I've always wondered whose idea it was to remix and extend the song. It's one of the best reworkings ever if you ask me.
The synth intro heard in the single and extended version is actually present elsewhere in the song and can be heard in the background of the album version if you pay attention. But whoever decided the song could be improved so much by remixing is brilliant :)
For the record, it was remixed and extended to about 7:12 for a 12 inch single first. This runtime includes a horribly edited-together montage of other tracks from the album at the end, kinda like the thing in "More of That Jazz". The actual remix of IWTBF goes til about 6:05.
Then the single version is actually edited from the extended mix. It's a unique situation where the single version ends up being longer than the album version.
You can hear the full extended mix on the Hollywood Records version of "The Works" or on "The 12 Inch Collection" however the 12 Inch Collection version runs a little slow for some reason which adds a few seconds to the running time.