Hi zoobuffalo,
the phenomenon you are experiencing looks like the intended behavior of leaderboards:
To ensure that every player can only have one score per leaderboard (which is the way leaderboards should work - you wouldn't want to have the player Frank appear 3 times in one single leaderboard) each game installation gets a unique ID.
Every time this installation posts a score the posted score is evaluated against the leaderboards settings: If the leaderboard order is set to High-To-Low, which means that higher scores are better, then the leaderboard checks if the current installation has already posted a score and if the newly posted score is better than the old one. Only then will the score be updated in the leaderboard. In other words: Player's scores can only get better, not worse. You wouldn't want players to overwrite their great scores with worse ones - angry people and all.
BTW: This unique ID is not the player's name that goes into the SDK's postScore method. Using a unique ID ensures that the player can change his name on the next score post, but still is validated against his previously posted score.
Finally, if you delete/uninstall the game from your (development?) device and reinstall it (or if you purge your browser's cache), the game has to create a new unique ID. Score posted from the new installation will be treated as a new player and appear side-by-side with the old player/previous installation.
Is this of any help to you?