This article describes the difference in the statistics between WAN Memory Storage Partition and WAN Memory Disk Utilization.
When you go to Configuration > System > Setup > Storage tab > WAN Memory partition, it gives some values for how much space the WAN Memory partition is free on this partition.
If you go to Configuration > System > Diagnostics > Acceleration tab > WAN Memory drop down, you get some WAN Memory Utilization statistics. This gives you the statistics in relation to the WAN Memory disk store. The WAN memory process stores data segments in the disk during cold passes when accelerating.
These two values are different because the Storage Account is for everything in the partition, including all files in this partition, and not just the disk store. For example, if there is a disk store that is only 50% used, the partition will still add the whole 100% of the disk store, and all other files in this partition.