Auto-what?

Today I had problems with the auto-advance ("new style") of the AVL.
I had woven a little more than 50 cm, with (almost) no hint of a problem.
It looked like this (all pics get bigger if clicked):

As you can see, there is the occasional streak, but I'm sure that can be blamed on me. (I have found that it is important to keep the rythm, or slight irregularities easily happen.)

Anyway, I was weaving on, and on, and on... when suddenly, this happened:

I tried to remedy it the first times it happened, to no avail. Irregularities continued - the auto-advance really advanced, and advanced...

Something had to be done (and the scarf was ruined anyway). So I put a brown marking thread, and lowered the a-a arm some.
And then some more, and some more... Almost no change between adjustments to the arm.

What to do? Knowing it would be too much, but being almost desperate to "get a reaction", I put the arm at the very lowest point.
Wove on for almost 15 cm, when (again) suddenly I was getting almost double the pick count.
(I did not change anything at all between the two horizontal marks below)

Now I had to get the pick count lower, again... Again several tries:

After almost 50 cm fiddling around I had got the pick count back to where it was to begin with.

I started with approx 7 picks per cm, at the worst point I had about 3, came up to 12 and then back to nearly 7 (did not manage to get it below 7,3 - average over 10 cm).

What, why?!?

- I have never been able to understand why I can have the a-a give me (say) 10 picks per cm when weaving a cotton warp, put on another cotton warp (same grist, different colour) and have to adjust it to get 10 picks per cm.
I can accept (but not quite understand) that I will have to adjust it if I out on a wool warp - different materials have different properties - but, after all, the auto-advance device is purely mechanical?