I suspect some confirmation bias here. Usually, when people see two feminine women together, they tend to automatically assume they're friends. But the moment one of them looks more on the butch side of things, then they notice and they see a lesbian couple instead.

That being said, I've also once heard an argument from a lesbian as to why that may still occur more often than random: insecurities may be easier to navigate the more different people are in a couple. It would be more about minimizing envy than "filling a role" in the couple. That being said, I'm not sure how much we can generalize that from that anecdotical perspective.

There also may be other things going on, people are multidimensional. r/AskLesbians surely has more reliable answers.