I'm glad you are a sensible chap and stopped the engine when you noticed the fault. I don't think I always run it up after a belt change with the covers off, but I will make a point of it from now on, besides I like seeing the patches of white lettering whizz round in opposite directions..
Both belts appear to have run off the same amount, so it would point to a consistent manufacturing fault in the tensioners, I bet it's not very much though.