Understanding why MongoDB uses more RAM than the allowed wiredTigerCacheSizeGB

My mongod command

mongod --wiredTigerCacheSizeGB 5

In practice, I'm getting mongo instance to use even up to 8.6 GB (on a VM with 50 GB RAM) Indexes are using about 100 MB (checked with db.stats()) tcmalloc information:

MALLOC:     3776745552 ( 3601.8 MiB) Bytes in use by application
MALLOC: +   4744544256 ( 4524.8 MiB) Bytes in page heap freelist
MALLOC: +     20067616 (   19.1 MiB) Bytes in central cache freelist
MALLOC: +      3584128 (    3.4 MiB) Bytes in transfer cache freelist
MALLOC: +     12470800 (   11.9 MiB) Bytes in thread cache freelists
MALLOC: +     22544384 (   21.5 MiB) Bytes in malloc metadata
MALLOC:   ------------
MALLOC: =   8579956736 ( 8182.5 MiB) Actual memory used (physical + swap)
MALLOC: +     29933568 (   28.5 MiB) Bytes released to OS (aka unmapped)
MALLOC:   ------------
MALLOC: =   8609890304 ( 8211.0 MiB) Virtual address space used
MALLOC:
MALLOC:          21650              Spans in use
MALLOC:             59              Thread heaps in use
MALLOC:           4096              Tcmalloc page size

The server had been running for 3 days. How can I tell why mongo is consuming more memory than the allowed cache size? Is there any way to manually free up the "page heap freelist"? The command output is recommending Call ReleaseFreeMemory() to release freelist memory to the OS, but I don't think I can actually do that out of the process. MongoDB version is 4.4.1, I'm using mongo:4.4.1-bionic docker image.