Why is bonus modifier rounding so inconsistent and weird?

The Ritualist's 25% bonus can push a +4 to level of _ skills on an amulet to +5 to level of _ skills, presumably because it hits a whole number of 5. However, a +3 does not get pushed to +4 even though it's reached 3.7 after the bonus, which arguably should be rounded up as it's closer to 4 than 3, but it doesn't.

Then, counter to this logic, Sanctifying an amulet can cause +3 to go to +4 even though it's only up to a 20% bonus. So, it will turn 3.6 into 4, when the Ritualist bonus can't turn 3.7 into 4.

This just seems stupid to me. It's inconsistent for no real reason. Why is it like this? Why not just establish consistent rounding rules for bonuses?