another explanation is the difference in the Old market day which, in the US, was the CGO and NY operating hours (say, for most markets the hours between 8:00a.m. and 4:00 p.m.) and the closing prices including the now-called extrended hours of trade.
The markets now trade almost 24 hours a day. So, the difference in the 2 closing prices could be the additional trade in the extra time.
It is interesting to maintain the charts of the 2 various methods because when one chart looks confusing, the other may be clear.
icd