What would cause a set of jugs to be out of round?

[Kny21]

Have a set of jugs that needed boring and my goto shop stated that the jugs were 'out of round'. They were able to clean them up sure enough without basically taking all the life of the sleeves away thankfully.

But since I haven't really read or heard much on this subject so I was wondering what others with more experience may have to offer on the cause of this issue of the cylinders ovaling?

 

[Blue]

The rocking of the piston when its at the bottom of the sleeve and it starts to go back to the top and again at the top of the sleeve when it heads back down will cause the cylinder to wear more on the intake and exhaust side than they do on the front or back. Its typical for them to be out of round a little by the time there ready for a new boring.

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