Trouble with exposure using aperture priority

[neuroanatomist]

I would look at the exif data, I think you will find the flash didn't fire on the f/4 sample. Flash on this kind of subject can be tricky.

I think the flash wasn't supposed to fire at all - it's a T2i with the popup flash down. To me, the scenes in the attached shots look similar enough that they shouldn't meter 1.5-stops differently.