How random is the map selection?

I don’t have a very large sample, only the small number of games I have (around 80), so I wanted to throw in the question. In quick match or ranked, there are a number of maps available… But somehow, I have a very unbalanced feeling, how maps are “drawn”.
4 out of the 5 placement matches for ranked were on Hill and Dale… ok for me, but what are the chances. Next day, I play three consecutive games on Altai only to not see the map for weeks again.
Is somehow the likelihood of specific maps changing daily/weekly, or is it really pure random, and my sample size is just small?

The two maps I get are hill and dale. And mongolian heights.