Alternator/Aircon belt skips about

I have just changed the very very very long alternator/aircon belt on my Multipla 1.9 JTD and watching it go round afterwards I have noticed that it skips about 1mm to the left and then to the right on the central static pulley as it snakes around. The belt is on fine, all the V bits are in the right grooves etc. This is something I have not noticed before on any car and I am sure it is not "normal". The pulleys all look aligned from above, had no play and there is no new noise. The belt is on the right way round too...

Do your belts jiggle about too ?

I suspect my tensioner or the static pulley is duff and off centre, but I can't see anything. Or a deformed belt perhaps.