Why Human Fighter 6'th level then dual to 9 levels of Thief can gain 5 proficiency points in slings but Human Fighter 7'th level then dual to 8 levels of Thief can gain only 4?

How to reproduce:
Create party of two characters
Humans, Fighters, 18 in str, dex and con then points left to cha, put 2 points in slings and 2 points in short swords
Using console level up first one up to 6'th level and second character to 7'th level
Then dual both characters to thieves
Then using console give them 161000 exp points and level them up

The first fighter that was level 6 can train slings up to Grand Master of 5 points
The second fighter that was level 7 can train slings up to High Master of 4 points

What is the reason behind this behaviour?
Which mechanics make it work in such a way?

You are probably right about this one.
I've just tested pure fighter and at level 8 he gets 4 points of proficiency in total.

And The Baldur's Gate Wiki seems to confirm "Fighters dualed to other class may continue to advance in proficiency up to 5 once they regain their abilities. " Although it seems the level of active class counts instead of adding together levels for the purpose of the check when dual-classing.

Thank you for support