Simply because he was not drafted here, and plays on a line above a player that was.
I think it's more because his points last year were dependent on a higher than average shooting percentage, so a lot of people were expecting him to revert to norm. The problem with that line of thinking is that it ignores all the other variables in the equation, like shot volume, role, linemates, system.
Last year in 63 games with the Rangers, Strome had 18 goals on 80 shots, a 22.5 shooting percentage.
In 10 games this year, Strome has 3 goals on 13 shots, a 23.1 shooting percentage. It's a smaller sample size, but thus far he's getting roughly the same number of shots per game and scoring at the same rate.
Of note is that he has 7 assists in 10 games, where last year he had 15 in 63 games. Is that the result of more top 6 minutes? Is it the result of more time spent at center or with better linemates? Maybe he's just an all around more confident player and it's showing in his game.
I'd like to see him shoot more--he had 179 shots in 80 games with the Islanders in his 2nd season--but as long as he's getting assists, it's fine. Maybe he has found his groove and he's right where he needs to be.