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It is nothing to do with the "last modified" value of the physical file. It can't possibly be that because of course you can have multiple book formats associated with a single book record. Instead the modified column represents "mostly" what you asked for in your first post, of a date when that book *record* was modified by editing of the metadata, adding/removing formats or conversions in calibre. However since the other things you mention like "last sent to device" or "last read" do not actually modify a book record then of course they will not result in that modified date changing.
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