Well, I know this is sort of going 'down the rabbit hole', but I think it's also possible that regular season record isn't the best measure of which teams are the strongest going into the playoffs.
Some teams have lost guys to injury, some have had guys come back from injury, some have made key acquisitions at the trade deadline, some have played much better in the second half, et cetera.
So maybe the best measure would be the 'power rankings' or betting odds going into the playoffs, which presumably would account for these things.