I finished a round of standard singleplayer Junkyard, and it appears that I hit a button too quickly and got a four player game going before the score tally screen appeared. Unfortunately, it was my best score and didn't appear to save. Also I got a four player match going while in single player? I've attached a few screens.

Shows the four player game going:

Pause screen of this game shows:
a. The fact that it's a single player game
b. The current score I'm now "shooting" for, which was my actual high score.

Shows the missing score from the leaderboard: