How can 9 Purple Worms give enough XP to level from 19 to 20? Nine CR15 creatures grant 117,000 XP (9 x 13,000). It takes 50,000 XP to level from 19 TO 20. So unless the party counts only 2 PCs (117,000 / 2 = 58,500 XP each), that would be insufficient.
My guess is that you award the budgeted...