Seven levels of spells for clerics etc. is not a mistake, nor do the seven levels "map" to the magic user's nine. A daemon's magic resistance is not affected by a seventh level illusionist spell in the same way that it is by a ninth level magic user's spell, and a globe of invulnerability leaves an 18th level cleric with 3 spell levels while her archmage counterpart has 5 levels of spells which can penetrate it.
Rather than discard this as an error, I think it's more interesting to examine the questions about why this might be in terms of the game world. Because there's a potentially dark subtext to the arrangement, which could be focused on by a DM.
Illusionists are limited to seven levels like everyone else, and on the surface of it this is explanation enough - the magic user is simply better at magic. But that doesn't actually explain why clerics don't have higher level spells.
The cleric's spells come from deities that themselves often have access (according to D&DG) to 8th and 9th level spells; why do they not provide them to their servants? Well, do deities actually want their worldly representatives to have the ability to fulfil their own wishes, for example? Maybe the characters' gods are jealous gods. Perhaps there's something in the history of the game world which explains the limitation on clerical spells. Maybe the gods just don't trust mortals (and maybe that's because the gods were once mortals who have become gods). There's lots of possible answers.
|Bloody magic users; can't leave anything alone.|
There's potential for a campaign-wide tension there which can supply ideas for high-level play and NPC motivation and a bit of inter-party friction without having to go as far as the rather badly thought-out barbarian rules in UA.
In my 1200AD England setting I looked at this issue from another angle - the mediaeval Jew in Christian England. I wanted the Jews to have a similar role to their historical one; I wanted to retain the idea that they had in some way rejected the Christian messiah but how to do that in a gameworld where clerics commune directly with their gods and can simply ask "is this man the messiah?"
The idea I went with was that these Jews simply did not believe in an immanent deity. They dismissed the beings that the Christian clerics spoke to as demons and devils. They, in fact, rejected the whole superstructure of the AD&D clerical class (something I'm often tempted to do myself).
|And that's number wang! You've won a|
one-way ticket to Hell! Thanks for playing.
This split between clerics and magic users was quite inspirational and suggested many ideas for scenarios, patrons, magic items, and motivations for both small and large-scale events in the gameworld.
Some early D&D publications did try some of these ideas, if my fading memory serves, but it was mostly washed away in the name of party-balance: the ideal party having a representative from the four main classes of fighter, magic-user, cleric, and even thief (itself a source of friction). Perhaps it's worth looking at it again and seeing if something more can be made of the superiority of magic user spells over clerics and the potential rivalry between arrogant inquirer and those who wait patiently for the revealed truth.
One Other Thing
It was suggested to me many years ago that there was a default assumption that any spell in the book not in the magic-users' list could be researched at no worse than two levels higher than its level in whatever other list it appeared in. I have no idea if the person who told me this pulled it out of thin air or read it anywhere "official" but it does have a little support (gate, astral spell, phantasmal force, prismatic spray/sphere sort of, animate dead etc.). Interestingly, the overlap with druids tends to actually favour the magic user.
"Official" or not, it does raise the possibility that a determined archmage could completely usurp the powers granted to the cleric and research earthquake, heal, blade barrier, or even creeping doom. What then for cleric/mage relations?