Free space and photo-size in iOS5 differs between iPhone and iTunes?

Hello,

strange thing happening to me:

- before i connect my iPhone to my Mac iTunes sais there is 2GB free space on my iPhone.

- when i connect the iPhone it goes up to 5GB

- some time after the sync it goes down to 2GB again

---> the iphone says there's 5GB free all the time

i recognized that the only thing that is changing size is "photos".

Any hints? Does somebody see the same thing happening?

Thanks.