I'm running zfsonlinux and htop is counting the ZFS ARC (memory) cache under normal used memory.

This is because ZFS doesn't use normal Linux memory cache but it's own version.

It would be useful if htop could detect if ZFS is running (maybe checking for /dev/zfs) and in that case show the memory used by ZFS cache in a different colour or maybe joined with the normal buffers/cache.

ARC used memory can be checked in /proc/spl/kstat/zfs/arcstats (size):

# grep ^size /proc/spl/kstat/zfs/arcstats 
size                            4    3533162592