It is simple, as CC says, the lies about his 100% against Goku SSJG and the latter believes him, despite seeing its "10%" against Vegeta SSJ2 (Enraged). The truth does not surprise me that 10% of Beerus equal to its "maximum power" against Goku SSJG. The real power of Beerus is still unknown. Beerus (100%)> Monaka> = Champa> = Goku / Vegeta SSJB (Universal Tournament) >>> Goku and Vegeta (When Champa goes to Metro Beerus) ~ Golden Freezer (100%)> Goku / Vegeta SSJB (against Golden freezer)> Golden freezer (Cancelled)> Goku SSJB (Cancelled against Golden freezer)> / >> Goku SSJG (100%) = Goku SSJ (Post God).