Earlier today after one of my games, the game awarded me a bonus 3000 experience with the description “freshness bonus”. Under what circumstances does the game award this bonus?
In junction to @Vemonus’s answer, the wiki adds a bit different wording plus a nice graph.
A Freshness Bonus in Splatoon 2 refers to the proficiency you’ve earned for each individual weapon. This is different from Judd’s coin bonus from the original game.
Each weapon has a freshness rating that will improve every time you win a match. It starts at 0 and goes up one point each time you win a match. Losing a match will subtract from your score, but how much it subtracts depends on your current rating. Once you reach a certain amount, a point bonus is applied.
Take note that while your ranking will continue to go up, the highest you can go is SUPERFRESH! along with all three one time bonuses.