Shape size changing when rotated or using shape builder

The weirdest error ive encountered but its not just changing the WxH to opposites when rotating. I'm holding down shift to rotate and its not actually rotating to a full 90º.

I'm also noticing when i use the shape builder to cut a angle off the rectangle (im making a line at -115º over the rectangle) it also goes from being for example - 12mm high, to having the corner points slightly off and becoming 11.97mm but then if i have another shape itll be now 12.36mm - why is it being so weird with the measurements?

I've reset the preferences as well but nothings working