Actually it's the core of staples that never changes. If you go to see them tour after your, you do get a rarity here and there and a different set each time.
Sure, the "rarity" is still a moderately famous song, and not a tune that not even the band members remember anymore but the hardcore fan knows from memory. And sure, here and there you'll find head scratching choices (Why always bringing back Wrathchild? why no Still Life for Maiden England? why no 22 Acacia Avenue for the early days?), but in the long run, you notice the diversity in the setlists.
I mean - you go to a show, you get The Trooper, The Number of the Beast and 2 Minutes to Midnight, and Wrathchild if you're unlucky
you go to see them some times over the course of 2008-2014, you get to see Powerslave, Rime of the Ancient Mariner, Seventh Son of a Seventh Son, The Clairvoyant, The Prisoner and Phantom of the Opera. Without counting the best songs from the new album they happen to throw in.
And as Stadler pointed out, there's no going around it, it's always those same songs that get the biggest reaction. At my show in Italy Sign of the Cross and ESPECIALLY The Clansman went down very well, but the earth-shaking, the-walls-are-coming-down roars will always be reserved for The Trooper, The Number of the Beast and Fear of the Dark.