576k comes from 64k + 512k. The 512k is separate from the 64k. 64k is main memory. If you wanted more, the next logical step up would have been 1088k. If you have seen any XE upgrades they tend to be 64k + a multiple of 2^n(256,512,1024k) so the total would be 320k, 576k, or 1088k. warerat's design follows that trend.
BTW,
Why do you want 640k? warerat is not trying to build a PC clone. I have only seen 640k significant there.
Now it makes since. I see the trend your talking about, yes. I would have just thought that that would have been the next step, 640k, but I guess I'm wrong.