This happens to me as well; I suppose it's mainly because book problems are harder, therefore demand a longer time concentrating to solve them and so one remembers them.
Most tactic puzzles on chess.com (at least the ones i've solved so far) chiefly intend to make you familiar with tactical themes and develop a quick eye for spotting them in your games (since they're also timed). They're usually fast, one-or-two combination moves. In contrast, I find the book ones far more challenging... maybe in part because many are taken from actual games.
A few days ago I read a book of tactics, and I noticed that at the end of a day I remembered more or less the exercises that i had done. Without discriminating or underestimate chesscom t.trainer , certainly chess.com trenener is a useful tools .But when I finish to use every thing is gone from my memory .
First I Thogth to be me , but chatting here an there I realize to not be the Only one. Am I the only one ?