why're there so many threads in 2.8.4

i have a program just run 10 sence.

when i create sence , i set both internalThreadCount and backgroundThreadCount as 0;
and set the flag as followed

sceneDesc.flags &= ~NX_SF_SIMULATE_SEPARATE_THREAD;

why were there 5 threads when i ran the program?
what about the other 4 threads?