The "Modified" column in a document library reflects the date/time that the properties of the document, i.e. any of the fields in that row, were edited (NOT the modified property of the document itself). When you right-click the document and select "Edit Properties", your edit actions determine the date/time in the Modified column.
The Explorer view reads the actual modified property from the document itself. That's how they differ.
HTH,
Phil H.
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A virtual machine ate my accounting data. I transferred it to a physical box, then I beat it to smithereens with a sledgehammer. I feel better.