That's because your thinking is stuck in 3e mode, where everything is additive and a "10th level character" might not have any actual levels higher than 3, as in Ft-3/Wz-1/Rg-2/Cl-2/Bd-2
It shouldn't be that way.
First off, 5-class monstrosities like my example simply should not exist, period.
Second, if the levels advanced independently of each other and a 9-1 was thus only trivially different from a single-class 9, your issue would go away.
This is trivially easy to work around. All you need is some guidelines for how one might pick up a class during one's career - I'd suggest it would involve at minimum a half-year's training (i.e. adventuring downtime) to become a raw 1st with 0 XP in the new class. Then, you carry on as if a 1e multi from there.
Example: I start off as a straight Fighter for the first 5 levels (say, 20,000 XP worth), then decide I want to pick up a bit of MU on the side so I can do my own Identify spells on my armour and weapons. I take 6-12 months off from adventuring and spend this time instead in a crash course in spellcasting*. When I come back I'm a F-5/MU-1 and my XP are split at 20000-0. I decide from here on I'm going to divide my earned XP 50-50 between my two classes.
Next adventure earns me a total of 6000 XP; so I'm now at 23000 on the Fighter side and 3000 on the MU side, which bumps me to MU-2: I'm now F-5/MU-2. And I carry on from there.
Point is, using 1e as a jumping-off point doesn't necessarily mean you have to multi right from the start.
* - some sort of lengthy training time is essential - this business of just jumping into an entirely new class on a whim when you bump (as in 3e) is so lame it can't stand up.
Lanefan