But the marketing strategy of music companies that embrace iTunes might make a big mark on lists like these over the years.
Before the mid-60s singles were all that counted. The youth bought more singles than they bought albums. This changed in the mid-60s and only a few singles were released per album. The album wasn't the compilation of singles, b-sides and leftouts anymore, but the singles were released to plug an album.
Right now with the digital distribution of songs through iTunes and Amazon, people are buying less and less whole albums, but prefer to pick out the few songs on it that they actually like. Several record companies acknowledge this change. They know what the strong songs on the album are and release them as 'digital' singles. They don't bother to distribute them in a physical format to the shops. They only announce that 'this' is a new single, make a cover for it and plug it to the radiostations.
For $0.99 you buy an hitsingle on iTunes. This results in an amazing flow of number ones for the Glee cast (a TV series in the US).
Comparing these iTunes number ones, with the number ones sold in real shops is like comparing apples with pears. Yes, they both fruits, but that's it.