I was just browsing through the play store and noticed that there are a few free apps in the top grossing section and that's a little puzzling. This might not be the best place to put up this question but I didn't know where else to go .
As per Oxford: top grossing means earning more money than other similar things or people.
But as per google app store it may refer to :
Top-Grossing = Sum of all revenue generated from the purchase of the app + sum of all in-app purchases
It also means that app listed in top grossing lists holds people's attention for the most time. So even if an app is free but still if it is getting downloaded the most then it will feature in this list.
Also many of the free versions apps like angry birds, cut the rope etc. provide in app purchase ability i.e you don't need to bye a paid version but to access more features you have to pay some amounts which ultimately boost's apps revenue and ranks app higher in list.