That's how you determine the most popular group by year.
Earlier I made a thread saying f(x) was arguably the 2nd most popular girl group of 2011 after SNSD but some users said T-ara was more popular cause of Roly Poly.
Digital sales means people only care about the song and not the group. So if we compare both f(x) and T-ara digitals and physical releases that year then we can see f(x) was clearly more popular.
f(x):
Hot Summer digital single: Over 3 million.
Pinocchio/Hot Summer combined Korean physicals: 144K
T-ara:
Roly Poly digital single: Over 4 million.
John Travolta Wannabe/Black Eyes combined physical sales: 86K
T-ara had the bigger song but f(x) was the bigger group.