NAS updated to 6.10.3 with OS re-install

I have a Pro6 running OS6 that I rarely use.  After 6 months of inactivity, I booted it to help another forum user.  There was something amiss with the OS (which was 6.9.6), so I did an OS re-install.  I was quite surprised to find it installed OS6.10.3.  Automatic updates are disabled, and it's set to check for Long Term Support updates.  In other words, I did everything I should have to prevent it updating to 6.10.x, and it did so anyway.  Oddly, not to the latest 6.10.4.

I have had legacy rack-mount units do that when I booted them when connected to the network and there was no volume.  Since this NAS could not boot, I suspect the same situation applies -- it updated because there was no bootable volume on the drives.  I was told before that it was "impossible",  Well, that's simply not true.  Maybe it has something to do with the fact that the boot system in flash is still the 4.2.x one and we simply need to be warning people about this.  But how can one know their OS won't boot until after it has happened and the update has already occurred?  What if they re-boot again in the middle of the update?

Anyone know what's happening?  I frankly don't care which OS is on this particular NAS, but I would be very unhappy in an update to a version from which I could not revert to 6.9.6 were to happen on my main NAS.