That's the ARC. The ARC max size will default to (at least on FreeBSD) total ram -1GB[1]. Because the ARC is read cache it will shrink if the system needs more memory. Unused ram is wasted ram.
That's probably the ARC, the in-memory caching structure, which gives up memory when applications need it. The weird thing on Linux (and maybe the others) is that the kernel marks the memory as "used" and not "cache", so tools like top/htop show high memory usage, even though ZFS will shrink its ARC when any actual applications require it.