That was my first guess, too.
But after reading the article by Bombich Software I am not so sure about that.
There should be Preboot, Recovery and VM (all of them hidden), system and data (forming a volume group, and both of them showing up in Disk Utility) and potentially another volume if somebody has created one inside of an APFS container.
I am confused.
I just checked my Mac Mini. Disk1 is shared by 5 volumes. Macintosh HD and Macintosh HD - Data share one container. I have the VM with 2.15 GB, two “not mounted” volumes of only 752.8 MB and free space. Those two “not mounted” volumes should be Recovery and Preboot according to the mentioned article. So, yes, the third Macintosh HD volume looks weird.
I wonder what Disk Utility is displaying when @ThatNerd clicks on the greyed-out “Macintosh HD”…