It is a quirk of the math Larian uses, where they both add and subtract the (prof+ability mod) bonus, then add each of those bonuses individually.
The combat log should should read something like 1d20 + full bonus - full bonus + each individual bonus, which is the same thing as 1d20 + full bonus
e.g., combat log says: 7 (1d20) + 5 - 5 + 3 (dex) + 2 (proficiency)
= 7 (1d20) + 5 (full bonus)
= 7 (1d20) + 3 (dex) + 2 (proficiency)
All of the above are mathematically equivalent and result in the correct final roll of 12.
If you don't have proficiency in the skill, then obviously the equation changes. Something like 7 (1d20) + 3 - 3 + 3 (dex) = 10 (still mathematically correct, if convoluted)
If what I have explained is not what you're asking, then please post a picture of the combat log in question. Or maybe someone else has a better/more relevant answer.
Last edited by mrfuji3; 24/11/20 10:47 PM.