Well, for starters I've listened to Maiden for about 8 years, not so much anymore, but still shitloads up until a few years ago.
Despite the change in singers, it wouldn't be impossible to pick maybe one song from each album, make a collection out of it, and convince a Maiden-newbie that they're from the same album.
Some hardcore Maiden-fans can argue that Somewhere in Time has synths, SSOASS is more "majestic", X-Factor being darker, obviously the change in singers, and that they have 3 guitarists now, but lets face it, these are all fairly small traits that only people who listen to Maiden a lot actually thinks about.
They can play The Trooper, Fear of the Dark, Run to the Hills or any other fairly "known" Maiden-song on the radio, and people will say "this is Iron Maiden, sounding Iron Maiden"
Hell, even take any of the re-union songs, The Wicker Man or Rainmaker, it's Maiden sounding Maiden.
Yeah, they've tried changing things, but the small changes they did hasn't left much of an imprint on the world.
Let's face it, despite many hardcore Maiden-fans liking The X-Factor, I'd still say the majority of radio-listeners would bash that album over and over, not only cause the lack of Bruce, but because it was a bit different after all.
Since the re-union, they have become slightly proggier with more longer songs, but the guitar-playing is still classic Maiden, it might have improved during the years, but it's so familiar that you hear right away who's playing the lead solo, or whatever.
Bruce, still sounds very much like Bruce, and despite having 3 guitarists now, it's not much of a change, because to actually hear a very noticeable difference, you need to have a killer soundsystem, or just a very good ear, and that doesn't apply to very many of Maiden's fans.
Maiden have changed, but it's so tiny in comparison to everything else, that nobody really thinks about it.
In a poll like this, they have nothing on Metallica in terms of changing their sound, Metallica has quite a few songs on Load and Reload, where I think many people would go "THIS is Metallica? doesn't sound like them at all", but that isn't the case with Maiden.
Even if Bruce is not singing on a certain album, instrumentally it's still very easy to recognize, through distinctive guitar solo's and such.