Hi Sandra,
Windows Embedded Handheld 6.5.3 (or Windows Mobile 6.5.3, they've the same architecture) is based on a Windows CE 5.0 Kernel that has a limit of 512MB physical RAM memory. The OS cannot access more than 512MB even if it is available like on the MC67 high memory setup.
Windows CE 5.0 Memory Addressing

So, you're getting 312MB of RAM because the physical memory the OS can see is only 512MB and then there's the space occupied by the OS and the device drivers.
Regarding the 4GB of Flash, I'm not sure if this is because we're using FAT16 that has a partition size limit of 4GB or for other reasons. I've to double check on this.

The reason why we release the device with 1GB of RAM even if is not fully visible under Windows, is because this is the minimum amount of memory that we believe can provide a good experience under Android.
This particular MC67 part number is mean to provide an easy upgrade path from Windows Mobile to Android as it is upgradable to Android (sending it back to our support).

Best regards
~Pietro