If the model units are metric, and you enter imperial dimensions as ft, in and fraction values, for example 1’8 5/16, these can be mangled on entry, and produce unexpected and wrong results if you use just feet followed by a foot mark (') then inches and fractions without the inch symbol (").
It’s been partially corrected in SU2019 onwards, I think, so if you use both ft and inch quotes, you get the right answer. So 1’8 5/16" produces the right length in mm, but 1’8 5/16 doesn’t.
See SU doesn't recognise fractional inches (e.g., 1/2") as an imperial measurement when units are mm