Hi Papabear.
There are two issues there :
- First one, starting when nearing D1-M3 level, is the lower amount of players compared to the bulk of them (which are in gold-plat-D3). It will be easy for you to say if that’s the case, because the displayed MMR of your opponent will be farther away of yours than the usual, and the waiting times will increase proportionally.
- Second, is that there are people who deliberately lower their level in order to be matched with players of lesser skill. Those are called smurfs, and are of course much stronger than what their displayed MMR would imply. Depending on the leagues, there currently are between 20 and 25% smurfs, sometimes more in some specific tiers.
This induces that depending on the ratio of smurfs they’re used to play, the average regular players will also be of different strengths, since an user who’s used to play at hours with ≥30% smurfs and manages to maintain his MMR will in fact be stronger than the ones who play at that same MMR but at hours were there are only 10% smurfs. Also, as Dallarian says, one user trying to move from one playstyle to another will underperform during a certain time.
And so the level of play can vary wildly in a same league even among regular players. That’s even more the case for diamond, since MMR-wise it’s one of the largest leagues. 
Welcome back ! 