Yeah, most jigsaw puzzles do not have precisely the number of pieces advertised. Here's an amusing video (by the channel Stand-up Maths) that does a deep dive into it. https://www.youtube.com/watch?v=vXWvptwoCl8

TLDR if you don't have a half-hour: puzzles are usually cut with the pieces on grids, and not all aspect ratios are conducive to that with all piece counts. Like, you might want a 2:3 shaped puzzle with 500 pieces, and 18x28=504 is close enough.