Note that another app may have a better revenue/day with a higher average top grossing rank because revenue are not linear with top grossing rank and because it is an everage over the month. The goal is just to show -roughly- how the market is evolving.
Of course, we would need much more data if we wanted to analyze precisely the market (eg one doesn't know how much sales are on iPad and iPhone, the ranking are not the same so it is really possible to normalize,...). But it offers some insights. For example, if you compare 3/2011 and 3/2012 you see that there is a 66% increase in revenue despite the rank is really not good as in 2011. So yes the market is larger (ok, you already know that).
Montessori Crosswords US Revenue vs US Education Top Grossing Ranks.
Note that smaller ranking is - of course- better.