Win XP Pro SP3 4GB Physical Memory - Photoshop Elements v4:

Task manager shows available physical memory as 2.6GB. Photoshop preferences shows 1.744GB available memory. Why the difference?

Another computer running same OS with 2GB physical memory:

Task manager shows 1.5GB available physical memory. Photoshop preference shows 1.755GB available which is actually more than really exists. Why the difference?

On both computers Photoshop reports the wrong amount of available memory - one computer too much - other computer not enough. I am trying to understand how this works. Can someone help?