This has come up in other threads, but I've always been highly suspicious of the song writing credits in Maiden. Did Bruce really write Powerslave from top to bottom (when what evidence I can find suggests he's never really been that adept at guitar)? Does Steve really write every guitar harmony on his solo credit songs? I think Dave is more involved behind the scenes than the credits allude to.
Songwriting credits are, traditionally (and legally) given for lyrics, song structure and chord structure together with the main melody (chord structure by itself is not copyrightable). The point is, the song is nothing more than what can be performed by a single person on vocals and accompanying instrument (typically piano or guitar). Harmonies, solos, chord voicings, and sometimes even underlying riffs are not part of this and would traditionally not earn the contributor a credit.
Now, metal is not the singer-songwriter type of situation in which these guidelines were hammered out, and in the end the credits are registrered by the band as they like (unless a court decides to award it to somebody else after legal dispute), but if Maiden followed the traditional guidelines it would only mean Bruce at the very least wrote "Powerslave" singing to a single acoustic guitar - which the band then could have arranged for 2 electric guitars, bass, drums etc (to his wishes - or not!). Arrangement is different from songwriting, and does not receive credit in the same way.
The main point is, songwriting credits vs the final recorded version of a song
are not necessarily the same.
The underlying reason for why it is like this, is that giving someone a credit for taking an acoustic song and arranging it for a full band (or vice versa) without contributing to lyrics, structure and so on resulting in a credit, would mean that every single song either would have to be performed exactly as written without any room for interpretation, unless the contributor to the arrangement should also receive credit. If they do we suddenly have two different versions of a song with different credits, but how do we know who should get credit for a third version now? It's legally impossible to have it any other way.
The song is what is able to be identified as that particular song. No less, no more.