Question about Unique visitors count

Hi folks!

Everything fine with matomo, thanks to Matomo dev team for nice product. Only one thing which I can’t understand how Matomo aggregating unique visitors count. For example, we use log import feature (we are importing web server logs to Matomo application), if I take first month when we start using matomo I see about 20 millions unique visitors for that Month.

Question 1. I assume, since we just started use Matomo, probably all users are unique except those who visit site again during that period?

Then every month this number decreases, for example 7 months later I see 7 millions for same period “month” with bigger visits & pageviews count.

And finally, If I take last month (August 2019). I see about 2 millions for period “month”.

Also, if I take period “day” or “week” for same month, for example August 28 I see probably same number ~2 millions. Can someone explain this?

Question 2. So how to interpret month results? I suppose month results = unique users sum of all days of given month?

Question 3. Why unique visitors count was so big on first months when we start use Matomo?

Thank you in advance!