One scenario would be if it scanned it once in 8 days and once in 4 days, due to, say, an elliptical orbit or something. Thus, twice in 12 days, but not once every 6 days.
Alternately, complex orbital tracks may result in irregular accumulation of multiple scans, with double scanning only finally being achieved after 12 days.
Alternately, complex orbital tracks may result in irregular accumulation of multiple scans, with double scanning only finally being achieved after 12 days.