Professional bowlers bowl on 'sport bowling', or 'sport shot' lane conditions, where the oil is applied much more evenly to the lane, in comparison with what recreational and league bowlers bowl on, known commonly as a 'house condition', or 'house shot'. On a 'house shot', the middle of the lane is routinely oiled with a MUCH GREATER amount than what's put down on the outsides of the lane, closer to each of the gutters, or channels. This is done for the 'house bowlers', so their strike shots stand a much greater chance to reach the strike pocket, even if they miss their targets inside (closer to the headpin), or outside (closer to the gutters). The result -- they get a 'funnel effect' - shots outside hit dry earlier, and hook back to the pocket as the ball gets down lane - shots inside hit oil immediately near the foul line and skid almost all the way to the pocket, preventing the ball from getting left of the headpin. Shots like these result in strikes VERY often. If this doesn't happen, then typically an easy, routine spare is left for the bowler to shoot at on their next shot.
Also, pros bowl WAY MANY MORE GAMES in a typical professional event during a week, than an amateur would bowl in their league/leagues would. As a result, lane conditions tend to transition more significantly for the pros during a tournament, than what a league bowler experiences. Plus, the league bowler typically has the 'consistent house shot' week to week, whereas the 'sport shots' for the pros typically change from week to week, as sport conditions can yield a vast array of different oil patterns, pattern lengths down the lane, as well as overall amounts applied. Lastly, the lane maintenance crew may or may not strip and re-oil between blocks of qualifying, match play, or final rounds (typically step ladder format), depending on the specific parameters or circumstances of the tournament.
So, to sum all this up, if the 'house bowler' is fairly accomplished and experienced, they can, and often do these days, post averages in excess of 200, with little to no coaching from anyone, or practice to help get there. If a particular 'house bowler' has EVER competed at an ELITE LEVEL, such as the PBA, and/or Team USA, they may indeed post an average that might exceed 240 or 250, because of this, plus todays' OVERTLY AGGRESSIVE REACTIVE RESIN bowling balls, which literally DRINK UP oil from the lane, and not PUSH, or carry oil down lane anymore, like their non-resin predecessors did.
As you now know the rest of the story, this would be why a PBA tour member has yet to break the 230 average mark for an ENTIRE year on the national tour.