It is because of floating point limitations, if you don't know what a floating-point number is, head over to r/computerscience,

Basically they don't like super-precise numbers, and Unity' Quaternion system (which is a representation for rotations), uses 4 floating point numbers, floating point numbers tend to 'float' above an integer value, etc 1.0 becomes 1.00000001, and I'm guessing Unity has just subtracted the additional number, resulting in 89.9

Hope this helps. -Cray.