Under normal conditions the number of connections is the same as the number of logical sessions (with node client "mongodb": "^3.3.2" and poolSize: 1).

Under load (more instances of the same script) the number of logical sessions in $listLocalSessions grows inexplicably and 370 connections will create around 3000 logical sessions in 5 minutes. Also the number of sessions grows alongside the load (the bigger the qrw stat in mongostat is, faster the sessions will rise).