Why are StreamingPool Memory and Texture Stats Memory figures so different?

Can anyone help me understand why these two numbers are SO different? Texture stats shows a max of 320MB for ‘fully loaded memory’ but my streaming pool is 1.5GB and Nonstreaming Mips is another 2.5GB. I’m trying to find out what’s in the streaming pool and I’m really confused.

[Edit: There are no sublevels, so it’s not ‘Current Streaming Level’ option on stats.]