I recently upgraded from the Canon 70-300 USM non-L to the newer 70-300 L version. Just to make myself feel better about the purchase, I did some quick image comparisons. And without a doubt, the L version is, not surprisingly, way superior in image quality. However, when comparing the images, I also noticed that the focal lengths were not quite the same. Both at 70mm and 300mm, the non-L seemed to be slightly longer in reach than the L version:

Non-L @ 300mm"
Click image for larger version.  Name: nonL.jpg  Views: 98  Size: 164.4 KB  ID: 1002

L @ 300mm
Click image for larger version.  Name: L.jpg  Views: 94  Size: 141.4 KB  ID: 1003

These are both on a tripod, about 7 feet away, on a 7D.
I don't know a lot about focal length math. Is this common between lenses for focal lengths to not match up precisely? Does it have anything to do with the fact that the non-L's front end extends more? I haven't had a chance to test these outdoors yet, but will this difference lessen as the distance from the subject increases?

Thanks for any info!