Some problems with mocha and scaling

Hi there! Mocha is behaving weirdly, again. I've got a very, very simple track, which works flawlessly in mocha, but when I bring it into AE, the scale is off. It's always a bit bigger, and i've also noticed that some tracks have some movement issues, as if the anchor point was in a weird position.

I think my workflow is correct. My main comp is a HD, 1980p, square pixel comp, and i'm fitting to comp and the precomping the screen to put into the tracked footage. The track works, but something in the scale is off. I can make it work by hand this time, but i've been draggin this error with most of my comps and I don't know what my mistake is. Should I put my anchor point somewhere specific before applying the corner pin data? I'm lost.

I'm uploading a screen to show the error, the blue background is where the screen should fit.