I think i need to explain the rules to be a bit more easy.

"To hit" rolls are calculated AFTER modifiers are applied. Example: To hit 10-2=8. In this case, the hit was not successfull. To hit 12-2=10. In this case the hit was successfull

"To hit (critical)" rolls are calculated BEFORE modifiers. Example: To hit 18-2=16. In this case the hit was successfull AND the hit is a crit, since crits are calculated before.