On the numbers going across what is the key element that keeps them lined up? Sometimes mine are lined up nicely, and then there is some up and down on some of them.

First things first, what's the car you are referring to?
I know on the w123, they are held aligned by metal fork tabs on the number wheels which engage to a metal strip on the housing, small plastic gears turn them.

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Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale

Dirt. It gets into the odometer and will put a drag on everything.

It doesn't hurt to clean the number wheels every few years and lubricate the gears.