<p style="margin: 0cm 0cm 10pt;" class="MsoNormal"]<span style="font-size: small; font-family: Calibri;"]I believe this has to do with four factors:
<p style="margin: 0cm 0cm 0pt 36pt; text-indent: -18pt; mso-list: l0 level1 lfo1;" class="MsoListParagraphCxSpFirst"]<span style="mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;"]<span style="mso-list: Ignore;"]<span style="font-size: small; font-family: Calibri;"]1)<span style="font: 7pt 'Times New Roman';"] <span style="font-size: small; font-family: Calibri;"]Resolving power, which is where those bizarre test sheets with horizontal and vertical lines come into play. Simply put, this is the ability of a microscope, telescope or lens to measure the angular separation of images that are close together. As cameras become increasingly powerful light sensors, the importance of resolving power grows. L lenses are supposed to have a higher resolving power, but not always.
<p style="margin: 0cm 0cm 0pt 36pt;" class="MsoListParagraphCxSpMiddle"]<span style="font-size: small; font-family: Calibri;"]Some lenses simply can&rsquo;t resolve the image exposed on the sensor sharply enough, especially cheaper and, in some cases, third-party lenses and particularly those specifically designed for cropped sensors (APS-C, etc.).
<p style="margin: 0cm 0cm 0pt 36pt;" class="MsoListParagraphCxSpMiddle"]<span style="font-size: small; font-family: Calibri;"]Magnify an image you know to be sharp. Look at how details, especially verticals and horizontals, can blur. I test the sharpness of my lenses by photographing a wall-to-wall bookcase and see whether I can read the book titles at various settings.
<p style="margin: 0cm 0cm 0pt 36pt; text-indent: -18pt; mso-list: l0 level1 lfo1;" class="MsoListParagraphCxSpMiddle"]<span style="mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;"]<span style="mso-list: Ignore;"]<span style="font-size: small; font-family: Calibri;"]2)<span style="font: 7pt 'Times New Roman';"] <span style="font-size: small; font-family: Calibri;"]Centre and corner sharpness. The optical elements in a lens will handle light differently in the centre compared to those refracted at the edges, especially at different apertures and on different cameras. Modern full-frame bodies are less forgiving than cropped-sensor cameras, if only because the image surface area is smaller (edges are excluded). Those highly technical reviews Bryan and other authorities like dpreview conduct are way over my head, but I know for example that my 17-40 fill focus sharply in the centre and/or corners only at specific apertures.
<p style="margin: 0cm 0cm 0pt 36pt; text-indent: -18pt; mso-list: l0 level1 lfo1;" class="MsoListParagraphCxSpMiddle"]<span style="mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;"]<span style="mso-list: Ignore;"]<span style="font-size: small; font-family: Calibri;"]3)<span style="font: 7pt 'Times New Roman';"] <span style="font-size: small; font-family: Calibri;"]Back focusing issues: under specific conditions or with slightly &lsquo;defective&rsquo; lenses, the image is not focused at the point where the sensor lies but slightly in front of it, resulting in a slightly out-of-focus image even though you&rsquo;re sure your AF is working fine and you&rsquo;ve taken every precaution to ensure image stability. In my experience, this happens more often with cheaper, third-party and zoom lenses than L lenses and primes.
<p style="margin: 0cm 0cm 10pt 36pt; text-indent: -18pt; mso-list: l0 level1 lfo1;" class="MsoListParagraphCxSpLast"]<span style="mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;"]<span style="mso-list: Ignore;"]<span style="font-size: small; font-family: Calibri;"]4)<span style="font: 7pt 'Times New Roman';"] <span style="font-size: small; font-family: Calibri;"]That&rsquo;s life! We&rsquo;re not perfect and Canon certainly isn&rsquo;t. Sometimes, whatever magic that&rsquo;s in an expensive, highly praised lens just doesn&rsquo;t work. Poor QC, production, who knows? I find Ken Schwarz&rsquo;s final remark very appropriate and all-too likely. I think it is possible to have several copies of a lens and see differences, which is why I prefer to spend more money in a shop where I know I can return a defective lens for a replacement than always looking for the cheapest retailer. I once returned four copies of an L until the shop gave me a decent copy. Moral is, always check out an expensive lens for vignetting, CA and other visible defects before parting with your hard-earned cash. Reputable stores let you test their wares first.
<p style="margin: 0cm 0cm 10pt;" class="MsoNormal"]<span style="font-size: small; font-family: Calibri;"]Hope that helps. As you can see from my shaky definitions, I&rsquo;m not much into the technical side.