Wow, really? That seems unusual - how are you defining "team"? I use it to describe a group of about 7-10 people and I have definitely seen many of them with no women.
20% are women in the places I have worked so you would expect there to be around 2 on average then. The probability then that a team of 10 has zero women would be around 10% so not that common.
20% is definitely higher than average, and higher than any company I've worked at. And you're assuming a completely random distribution, which is not the case in my experience, for whatever reason.