Yeah, the memory metrics aren’t super-easy to interpret, in general. You can see a better breakdown by looking at Memory - Detailed in the per-instance panels, and /proc/meminfo within the Machine itself is the gold standard.
A 256MB Machine has only ~210MB to work with in reality, and like you said, that 210MB still has to accomodate the kernel’s data structures, PID 1, hallpass, etc.
It’s actually saying 2,263,508 KB, which is >2GB. Misreading that line is a super-common error, if you look in the forum archives, 
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Also, just for future reference, the anon-rss field is the most useful one, in the OOM reports.