I think the internal memory is partitioned a little differently with that.
if you go settings>applications>manage applications...
On my phone in the Downloaded tab it shows I am 488mb out of 2.5GB available. Same number for the All tab.
If I go to the Media Area tab, it shows I am at 703mb used out of 7.3GB
Seems to me OS and Apps installed to their default location are in the same partition, and then the second 6mb partition is where you find the stuff in the "Internal Storage" if you use the Files browser app.
Since you can move apps to the "Media Area", I'd guess you have about 10 to 10.5g of area you could potentially install apps to, built into the phone.