I played many games of Aikido, Jousting or other mods. Let's say we have the Bushido Achievement (aikido), and I win a game of it, it's stuck at the "1000" number! Why?

the bushido achievement is for getting 1000 wins it is not a counter

The achievement system in funky a little bit. ;o If you don't get the achievement that's probably because the achievement is glitched and many others have yet to get it as well. So you are not alone.