5.1 won't drop in November. Be realistic -- when has a major content patch ever come out in the first month of the expansion? It won't drop until the last month of Q1, I can guarantee you.Because it's a better--but not by any means perfect--way of seeing how MoP is doing based on current subscribers over a period of time versus other expansions. What percentage buys into an expansion is more more meaningful than comparing MoP sales to how many units are sold when Cataclysm dropped with a far higher subscriber base. That said, no one really knows much of anything without a longer time frame to measure. MoP is a week, the rest were days. And you have a fair point in that there were more people to come back at the start of MoP than in previous expansions. That said, that same effect happens to some extent at the start of every expansion so it's something of a wash but not entirely.
In addition to this, while Wrath and Cataclysm came out during November-December, i.e. just before or directly in the holiday buying season while MoP didn't.
If you want to make sales comparisons, a much more interesting one will be in mid-January assuming that 5.1 drops sometime during November. A new patch (if it's good) + holiday buying and good word of mouth could leave things in a very good place at the end of the year.
Lastly, as I've mentioned a few times already, revenue/profits from sales will be higher with MoP due to the number of units they've sold digitally. Blizzard gets all of that versus the past where with a box sale, after expenses and retail markup they were likely getting 60%-70% of it. That's a big deal.