USBAXP is the dynasty's long-term scoreboard. Wins and losses reset every season. XP does not — it follows you, year after year, and it is the record of who actually carried the league.
XP is tied to you, not your team. It is keyed to your gamertag / PSN, so it survives a team change, a rebuild, even leaving and coming back. The system rewards one thing above all: showing up. Playing your games, logging your scores, holding your spot when the dynasty is five years deep and the novelty is gone.