I'm a new player but I noticed this too - I think it only applies when using full power, and the aim seems to vary by about a stone's width. I don't think the power is affected. Slightly below full power still gives the exact aim you chose.
I guess this was implemented because otherwise, any shot with full power would be too easily repeatable.
You can replicate full power every time with no timing element (just hold until release) - if the aim were identical every time, you'd be able to hit the exact spot you want without fail, and not even needing to sweep as you'd be 100% sure where it's going as soon as it's released.
The variation and skill in release timing at any other power level means it isn't necessary to 'RNG' the aim.
