MongoDB used Memory + Swap

Problem: My server is using more memory than I set in WiredTiger (wiredTiger = 27GB). It starts off using 20GB RAM, but increases to 48GB RAM after 7 days.
After 30 days it is left in 30GB of used RAM + 50GB of Swap.

What is the explanation?

After 30 days with 30GB of RAM in use + 50GB of SWAP I’m left with 34GB of idle RAM.

Can you guide me in this analysis?

Mongo Version: 4.4.2
WiredTiger defined: 27GB
Memory in Server: 64 GB/RAM
Swap in Server: 64GB