I think he has consistently hit when you normalize for small sample sizes. And everyone is supposed to show a little more power in AAA and the majors because of the new baseballs.
I have actually been criticized for saying his ceiling was still TBD between a 4th OF and an average regular with defense in CF being the really big factor. If his CF defense is only a little below average, he helps the Phillies even if he is a bit below the average regular level because he is here and does not cost the extra prospects and money a veteran would cost.
As to experience, low floor guys tend to move fast. The Reading slow start was more the aberration and tailing off in his first season at Lakewood was probably fatigue. Andrew Benintendi skipped AAA and while he was a better prospect than Haseley, the level of readiness from playing at a top college program was similar.
I also fully expect some dropoff the second time around the league. It happens to most players and Haseley still needs to learn the strike zone more (as most rookies do). When the expectations are not greatness, but simply competence, Haseley is the exact type of player who can quickly fit in under those circumstances.
One small thing on the surprise factor - I did not think it was 100% likely that he could do this well. I'd have put it a little better than 50% though. Williams should be a 750-800 OPS bat too so it does not always work out.