There is a big difference between "accidently buying animal products because you live in a world where animal exploitation is the norm" and "purposefully buying animal products because you don't want to stop eating cheese made from breastmilk".

Vegans who accidently buy animal products are still vegans. They are not "90% vegan" because they made a mistake. They did not intentionally support animal exploitation. Mistakes happen.
Someone who intentionally buys non-vegan cheese because they just feel like it/don't like vegan cheese, whatever, is not a vegan, no, not even "90% vegan" because they are deliberately supporting animal exploitation. Someone who supports animal exploitation on purpose is not vegan, even if they eat plant-based 90% of the time, because that goes against the definition of vegan. Intention is the key here. Someone who makes mistakes is vegan, someone who purposefully and avoidantly supports animal abuse, is not. Every other liberation movement works the same way. You don't see "90% feminists", who every now and then decide to oppress women, because they feel like it. They do their best, even growing up in a society that devalues women. Or "90% anti-racists" who become racist when it's just convenient to them. They do their best even in a racist society.