When Cache does not = cache

Cache date doesn't represent reality

[twebdonny]

What is occurring when a Google Cached page for a date specified is not a representation of the actual page on that date, but a cached page from much earlier?

[doc_z]

Unfortunately, this seems to be not unsual these days:
I don't understand Google's Cache Date [webmasterworld.com]
Cache is showing incorrect date [webmasterworld.com]