The phrase "whole other ball game" means that something is completely different or separate from what was previously mentioned or understood. Full definition
We finally began potty training about a month ago (he has a speech delay so he is a late trainer) and he does great for pee but the poop is a WHOLE other ball game. (strawberriesforsupper.com)
But white chocolate & peppermint is a whole other ball game. (justsotasty.com)
I'm not quite sure because I only ever adjusted recipes at 5000ft; 10,000 ft is a whole other ball game! (hummingbirdhigh.com)