That game has Shaman.
Quote+1 Action
+$1
You may trash a card from your hand.
In games using this, at the start of your turn, gain a card from the trash costing up to $6.
It's very normal that Estates get trashed by a player with Shaman and that the "In games using this" effect has the other player gain the Estates. While you normally have a choice which card to gain from the trash (and get asked about that by dominion.games), if there is only one card in the trash you have no choice and will gain it automatically if there is no other "start of turn" efffect.
That's why it's very plausible that you have 4 Estates (or even more) without ever concsiously gaining one after your initial 3. It was an automatic effect by dominion.games (which in this case obscured the fact that you gained an Estate).
In Shaman games I usually have the trash permanently open, to see what cards await me.