Programs were just an example, but this applies to humans as well. Say, you bring a kid that just learned chess to her first Chess960 game, with pieces shuffled, and claim it's still "chess".
Will she be surprised whenever you play some weird castle on some move? If so, the game on her head (chess) and the game on your head (FRC) were different. This isn't unlike me inviting you to play a game of "chess" but on a move out of nowhere I drop a Knight on the board and put you in check, and you're surprised to discover we're playing a variant, which is not chess (like FRC is not chess.)
The part where it diverges is when you have to explain to her "the difference between this and chess is these weird castling rules I'm going to explain", and you have to proceed to explain them and give examples. If this was chess this wouldn't be necessary.