The root cause of the issue is that WG new web interface does not include some beta tanks, such as KV-220 action and M4A2E4 Sherman in the tanks list, but it still includes battles played in them in the total battles and other stats.
You can check whether you are affected by either comparing your in-game tanks list with your WG web feed tanks list, or by looking at the WG web feed tanks list sub-totals (under "Vehicles", adding them up, and then comparing that sum with the "Number of battles" on top.
Also, have deployed a new version of NoobMeter which displays a warning for users affected in NoobMeter.
The calculated Performance Rating for players affected is going to be at least somewhat imprecise.
I and some other players have submitted a ticket to WG, so they should be aware of the problem and will hopefully fix it.