They’re cheapo ones from AliExpress. I bought 10m and cut all belts from that.
I actually had issues with it due to the thickness earlier in the build, I was struggling to fit them in the belt holders and for the X axis I had to buy a different belt altogether as it was too thick and I just couldn’t get it to fit in the side plate.
I think the typical belt thickness is 1.38mm and when dealing with it not fitting well I discovered this one is 1.51mm, so a little thicker than usual.
Possibly the thickness is not the only abnormal thing, the pitch or just general build might be, but as you say, ~10mm is a fair amount, so not sure. Also strange that only one side would be affected, because the other is spot on. (Unless they’re both off by a few mm in opposite directions 
)