that's an assumption then..
It's a pretty educated assumption.
Let's take a step back and think about this logically.
Hardware-wise, its highly unlikely that the Tegra2 SOC would have any trouble addressing 1024 MB of memory. This has been illustrated by the Notion Ink Adam, which has very similar internal hardware- including the same SoC and 1GB of LPDDR2 memory (same as Atrix) and has no problems addressing 896MB of it (like I said before, 128 MB is dedicated to the GPU).
Software-wise, there's no software ceiling from preventing the system from addressing 1GB of ram. You can verify this by running a test device with 1024MB device memory in the Android Dev SDK. Now we will run into the 4GB barrier eventually (since ARMv7 ISA is 32-bit), but that's another story.
There's no reason why Android will suddenly have trouble with 1GB of ram on a device. Unless Motorola intentionally locked it, but that's just evil.
But like I said, I'm fairly certain there are no software or hardware hurdles addressing the full 896 MB.