AFAIK, lactose intolerance is a spectrum. I come from a country with spotty lactose tolerance so I've known multiple intolerant people. Just to give you an example:

I am mildly intolerant. I can have a small glass of whole milk with some discomfort, namely gas and bloated stomach. People who are highly intolerant would have a really, really bad time with the same amount with cramps, diarrhea and whatnot.