How come the same song is often a different length on different albums in iTunes?

Different versions of the same song. This happens all the time, and not just in the iTunes Store.

And when it’s only by a couple of seconds, it may simply be the lead in and out times, which is just dead space.

None of the examples shown in your image are from ABBA’s 1976 Greatest Hits album, which is the album ABBA originally released it on. It doesn’t necessarily mean that it is a different version of it – as I previously stated, it could just be a different length of time for the empty space in the lead in and lead out of the tracks.

“Sorry, I'm a little confused, what do you mean by that?”

I assume you mean my Wikipedia comment… When you look up a song on Wikipedia, you are actually looking up a release it was on – usually the single it was released on, or the album it came off. It may give you all sorts of information about the song, but any times stated will be specific to that release. It sometimes mentions durations for other releases, also.

Wikipedia will be talking about a specific version.

Differences could be many and varied, with much of them barely noticeable to the majority of people. There will be no correct version, only an original release version and all others that follow.