When earning Truck Bucks, only the newly earned amount will be added to the leaderboard score, so they will usually not match the Truck Bucks you currently have in your inventory.

For example, let's say you currently have 1,000 Truck Bucks before beginning the event and you've just finished your first round and won an additional 100 Truck Bucks.

Great! You now have 1,100 Truck Bucks in your inventory, but your leaderboard score will be 100 points. Continue to earn more Truck Bucks to climb the leaderbaord.