Sir Charles Barkley
The Chuckster
Master Roshi is a technique person. Plus he's not a slouch either. He stays training, like Goku. So he uses technique where power won't work. Plus, the manga made it a point to point out that the Pride Troopers Roshi was facing were pretty weak. Also remember that, raising your power level instantly and bringing it back down again is an established technique in DBZ.
I just assumed as goku got stronger so did the rest of the team. Remember unlike the OG saiyans and freeza’s men... the DB gang possessed the unique ability to sense and measure another person’s ki. So when someone breaks a new barrier the rest of the team feels it and can use it as a measuring stick moving forward.