Yes. My guess is that the 2 outer DIMM slots were meant for a maximum of 512 MB each and the middle one is 1 GB max.
Usually dual channel RAM Slots in the motherboard are 2 over location wise. It would look like this on yours: (D=dual channel S= single)
D-S-D
So if you have it as 512-512-1024, the slot that the 1024 is in has a maximum of 512 MB so I'm guessing that it's only reading 512 MB.