automount: why filesystem umount is delayed if timeout has been reached ?

Issue

umount of automounted filesystem via autofs can take place after timeout has been reached, i.e. using default timeout of 300 segs :

/etc/auto.shared /test autofs rw,relatime,fd=18,pgrp=7733,timeout=300,minproto=5,maxproto=5,indirect,strictexpire,pipe_ino=1182763 0 0

Examining automount debug logs we can see :

Feb 19 12:30:29 rhel77 automount[7733]: mounted /test/otherfs
...
Feb 19 12:36:41 rhel77 automount[7733]: expiring path /test/otherfs
Feb 19 12:36:41 rhel77 automount[7733]: umount_subtree_mounts: unmounting dir = /test/otherfs

That's a total of 372 segs vs. 300segs timeout