Another distinction to make is business vs private users. Unless something has changed recently, Macs are basically unheard of in offices (aside from maybe the receptionist), so if 50% of the market is businesses where there's 0% market share, "how many people use a Mac at the coffee shop" will overstate their overall market share by 50%.
I not sure about the office bit though. In my office there are some macs although they are a minority. I'm also not sure offices account for 50% of the computers.
But yeah, many other variables to consider. Age as well, it's mostly younger people in coffee shops.
