Factorio Forums

In the "Interface" Tab, there is a setting that allows the player to set Custom UI Scale. The tooltip says any number from 75 to 200 is accepted. However, since the recent update, any custom values snap to the nearest multiple of 25. For example, trying to set the UI scale as 110% results in the game auto-correcting to 100%.

Yep, that's how it works, see https://factorio.com/blog/post/fff-277.