WordPress 4.9.4 is the first minor release of WordPress in over four years […] where not all users will be receiving an automatic update.
This isn’t by choice – a bug went undetected during the 4.9.3 development cycle, and was only discovered hours after 4.9.3’s release. The bug causes a PHP Fatal error to be triggered when WordPress attempts to update itself.
A side effect of this is that now (at least in theory?) we have data on how many WordPress sites are currently maintained manually.
(Update: Or not.)