The game uses MMR to match people up not SR. Furthermore what you are experiencing is likely a freshly placed tank (basically they have 7 or 14 wins) whereas you have more.
Now to explain further why that would happen. The system is placing you between 750-1000 SR below where your MMR says you are so that you can have the illusion of progressing back up to your “real” rank. However, your MMR is not changing so you are still going to face people at your “true” rank. What this means in your case is if I were to place Plat I would still be playing against Masters players because my true MMR is Masters.