DBZ is kind of a dumb anime like that. There aren't any feats you could actually judge their strength by because they only went all out against each other and Toriyama kept getting his own power scaling wrong. Vegeta was a planet buster at a power level of 18,000, and then Frieza's first form was at 450,000 or something ridiculous. After a few strength increases by multiples throughout the series we get to the Buu saga where apparrently they only have enough strength to destroy the earth 10 times. For such a popular show it was extremely sloppily done. I thought it was because his guard was down. He was still Super Saiyan Blue. Either way, he shouldn't have even been fazed by it.