I think a lot has to do with the microflora one has. The microflora can absolutely be influenced by diet but part is simply unique to the specific person. The specific bacteria one is exposed to at birth has a huge impact for the rest of your life as your immune system tends to attack bacteria that you didn’t have in your first weeks of life. Taking antibiotics can also permanently alter the micro flora. I know more than one family where one specific member has horrific farts but the others don’t despite basically identical diet. So it really can just be bad luck. Not everyone drinking protein shakes gets horrific farts but some do.