A piston travels up and down the cylinder and if there is insufficient spring tension on the compression rings the piston rocks and in severe cases the piston skirt will strike the wall of the cylinder, this is commonly known as piston slap.
Although there can be sufficient spring tension to ensure reasonable compression the rocking motion unfortunately releases oil from the upper ring and that oil is burned in the combustion process, leading to a loss of oil from the sump. This process is generally referred to as excessive oil consumption, because it was not the design intention that the oil would be released into the combustion chamber for burning. At higher revolutions the leakage tends to increase, as does the burning thereof.
The 2 stroke engine however, is intended to release oil into the combustion chamber for the purpose of lubricating the moving parts, as it doesn’t have an oil pump to lubricate those components directly, therefore excessive oil consumption would only exist if the oil mixture ratio or oil injection pump were not set correctly and there was too much oil being delivered to the combustion chamber.
BTW paragraph 1 is much the same for a bent rod