ok, well iTunes sales were $3.34 Billion in 2008. That's over double the combined RIAA figure of album and singles sales. http://tr.im/yZ7K I remember reading that Apple doesn't take a large cut of that amount if that even mattered here, so sounds like these figures are wrong, especially when other online music retailers are taken into account.
I think they probably consider buying a single track from an album and buying a 'digital single' (a small release with 1 or more tracks) two different things.
I doubt the former case is considered a 'single' in the traditional sense of the word in the music industry.
I think that's in millions (or billions) of dollars. Notice that there is also '1978: $8.1' next to LP/EP and '1988: $6.1' next to Cassette. I think they are trying to label the peak of sales (those black bordered sections) with the volume of revenue.
{edit} It says 'billions' at the top {/edit}