… Hmmm maybe, the 6th Feb (with my 30) was also a Sunday… 
I know the algorithm usually (sometimes desperately) sends you more than you send, even from my beginnings in Postcrossing, I sent 5 in first two months, but got 7 back.
and aslo after inactive and travel mode, your address is dumped into the pool of addresses due postcards.
In that batch of 30 I also got a number of consecutive postcard ID’s from some of the most active countries.
@HookedonPostcards did you get a number of postcards registered in sucession, then a large gap before you registered a recieved postcard?