Purely speculation but it seems like you get tiles based on what can fit on your map.

So if you have a river and a road leading to the same tile then you are more likely to get one of the more obscure tiles like bridges than if you didnt.
Ofc if you placed that bridge somewhere else then that bridge would still be in the tile pool for what you could get.
The more possible pieces that can fit on your map the less chance you have of getting what you want/need in that moment.

I've also notices that if i create a LOT of entrances that then i am offered more of the tiles that close routes.
I did a game on the winter map the other day and because the tool tip said boats move slower i closed off the road and focused on the rivers and by the end i think i was given like 13 river closers because i just kept splitting the rivers so the boats would arrive at different times (went element towers so no AoE).

Although sometimes when im LOW on entrances that sometimes it gets confused and cant figure out if i'm trying to close a route or to give me more splitters which can be frustrating as with less enemies you get less kills and with less kills you get less tiles and before you know it its been 7 days enemies have +140% hp and +70% armour and now you cant kill them anymore so you just wait 2-4 more days for them to slowly kill you.

The rules are very complicated in fact. Generally speaking, it helps with extending your paths and filling in blanks. But it also has randomness, so not all tiles will be ideal for you, and it's part of the strategical challenge to deal with that.