Thanks to everyone who replied, but sadly I still didn't get a full answer to solve this mystery.
In fact, the Head Damage is less important than the normal damage :
a weapon with 20 damage and 100 Head damage will make less damage in HS than a weapon with 100 damage and 20 Head Damage beacause normal damage are more important.
It's not very clear but the stats are a bit wrong...
Thanks for confirming my speculation; after I've made this thread I have tested more weapons to see if I can figure something out, and the only logical conclusion I came up with after testing is that the weapon's base damage is more important than its head damage because for some reason, a weapon with 100 base damage & 50 head damage would do more damage on a head-shot than a weapon with 50 base damage and 100 head damage on a head-shot, but that was just a speculation until you told me the same thing I came up with, so yeah, thanks.
But that opens up another question, which is why has "base damage" been made more important than head damage? why did the developers make base damage more important than head damage...? why is getting a head-shot with a weapon with 100 base damage & 50 head damage better than a weapon with 50 base damage & 100 head damage?
can you please contact the developers of this game and ask them about this problem/thread? thanks.