This may be a complete red herring .. in fact it almost certainly is, but ....
It is/was fairly common practice to machine plain rollers for flat drive belts with a crowned surface, ie a bit fatter in the centre.
As far as I'm aware, this is done so that the belt will self-centralise on the roller.
I believe this was the norm in old machine shops and mills where many machines were flat-belt driven from a common shaft.
Like I said, its very likely a red herring and probably doesn't apply to the monster tensioner rollers which I strongly suspect are machined parallel ... but it would be interesting to measure them.
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