Ever since replacing the cambelt and auxiliary belts, one of the alternator belts keeps somehow working loose & then starts squealing

Try as I might, I just couldn't get it to stay tight enough. In desparation, the other day when 'my' mobile mec' popped by I asked him to take a look. Straight away he said that the belts were the wrong size & were sitting too low in the pullies. He explained that it is the sides of v-belts were they 'grip' & that the teeth are there only to disperse crud & dust. The belts I'd fitted were 9.5mm pukker Volvo jobs.
Being me I had the old belts still. I got them out to find that they were 10mm! Contitech, ones I fitted in probably 2007.
So I contacted the Volvo parts supplier who confirmed that he had supplied the correct belts as Volvo only ever fitted the 9.5mm ones on 240's. There is no other Volvo option. He did though, kindly offer a refund if I wished.
I decided that I must try a set of 10mm, so have ordered some from GSF. They are on route.
Anyhoo' I was getting fed up with the squealing & yesterday I decided to refit the old Contitechs. Result was that both belts tightened & all is now quiet. The Conti's are not particularly badly worn, just past their 'sell by' date. So not in a major rush to fit the GSF ones, though I will do when I have a spare hour or so.
Here are the 9.5's just before removal:
And here the 10's fitted, you can see how much deeper the 9.5 sit in comparison:
Finally, the 9.5 Volvo ones after only say, 1000 miles :
Maybe the pullies become wider with wear, or was it because I fitted 10mm (supplied by Braydons Volvo Specialists) which have worn the pulley grooves wider

Funny though that all the aftermarket suppliers I tried supply only 10mm belts.