Shape size changing when rotated or using shape builder

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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