It looks like the round_up part of the formula may not be rounding up but rather rounding properly, i.e. numbers below 0.5 are rounded to 0, not to 1. I'm guessing the March 16 rating "matured" today to 0.5+ of a month and added 1 to the rating.

Calculations from poking around some random users:

https://bitcointalk.org/index.php?action=trust;u=452769
2019-03-07 rounding up should make this a +1, rounding properly would make it a +1
2019-02-23 rounding up should make this a +2, rounding properly would make it a +1
2019-01-30 rounding up should make this a +3, rounding properly would make it a +2
Actual rating is +4

https://bitcointalk.org/index.php?action=trust;u=1339716
2019-02-05 rounding up should make this a +2, rounding properly would make it a +2
2019-01-27 rounding up should make this a +3, rounding properly would make it a +2
Actual rating is +4

And so on. Note that I'm using a custom trust list so the ratings you'll see on those users will probably be different.