Sure let me call my hacker friend to get into your computer and know what you mean instead of you giving is the details!
That is me being a jerk but honestly you aren't putting effort to help, you said "decrease decimal function", there are literally over a dozen rounding functions, not even counting other features, here is a some https://exceluser.com/1837/overview-of-excels-rounding-functions/
I'm going to assume you mean the two icons on the home tab, this is just a way to show less of the number BUT it is still there, the reason it is inconsistent would be due to floating points in computers, here is a link about it if interested: https://en.m.wikipedia.org/wiki/Floating-point_arithmetic
But honestly, i think it is VERY unlikely that it would happen to you with 2.45 but I could be wrong.
You can increase precision on option IIRC, but to avoid this, i suggest to just use proper functions like rounddown().