Your highest round changes when you play private match.
Say you get Thirty in a game you searched. then 32 in private match.
then you play a private match again and go 20. your leaderboards will change to 20. Unless you search zombies, your leaderboards show your number of rounds from the last private match you played.
So if you have a high round on a searched match, and dont ever play private match, your leaderboards wont ever change
Please +REP for this, i gave the real explanation, plus typed with my PS3 controller. :]