more of a camera to camera thing.
you can take all the cameras you like and all the lenses you like, and you'll never find exact replicas of results.
I read something on this about lenses and apertures and ISO settings, etc. there's no such thing as perfect.
The 2 sensors are different designs from one another, the T1i, shared the same teck as the XS and XTI or whatever they were. the 7D is a newer sensor, this may just be the difference.
It could also be the different camera bodys focusing different. If you micro adjust the lens to the camera, you may get different results as well.
Welcome to the 7D and fun microadjustments
or a cloud passed by a blocked a tiny bit of sun....