It is something that is not one of the other listed items. Apps is a listed Item, but the data that applications accumulate is 'other.' Audio is listed, but a local audio attachment that may be downloaded to your phone is 'other.'
There is a new facility in iOS 5 to show the data that is used by applications. Take a look at that.
Also, previously I have noticed that iTunes factors into the calculations, items that are pending but not yet transferred, This should be much more accurate if you have implemented iCloud because the phone will always be more up to date. But because of the predictive nature of this (pre iOs5) the total space used occasionally became out of sync numerically, but not in reality—This simply due to the sync / download sequence nature of things.
And sometimes 'other' can be a bit mysterious because there are so many individual items to account for. e.g. I, Pod–cast a lot. I use much more space than music and this always adds 'other' overhead to that space calculation.
So, sit down with a calculator and and things up. Look at the detail of what you have flagged for transfer and look at the data allocation per application and you my be able to find the discrepancy.