1000 songs fits in 5 GB and 4 GB

I just realized something as I was watching the original iPod ad (for the original 5 GB iPod) from this digg link: the new 4 GB iPod nano shares the same slogan as the original 5 GB iPod – "1000 songs in your pocket." Was marketing's math off? Was Apple running low on new slogans? Or is there more to it?

The only element I can figure that makes sense out of these numbers is the introduction of the AAC codec and mpeg-4 format that Apple provides in iTunes and uses for all the iTMS music. It's been a while since I've done hard-core file size comparisons between the traditional mp3 and aac formats (I converted my entire library to aac for the quality a while ago) but I remember the size difference being something to take notice of.

Anyone else have any theories on this? Has anyone done the actual math to find out if Apple needs to lay off the sauce? Please feel free to weigh in, as 1 GB of space is no small matter.