First of all do not try to use 512 in your Universe Setup in Xlights. It will not work with this controller, you will get pixel color shifts when you cross Universes.
I just want to save some of you some grief when you have weird things happen.
This is normal and a by-product of DMX which is not evenly divisable by 3. As such, only 170 pixels can fit into one universe and you end up with 2 left over DMX channels (511,512). The controller either has to use them up as in:
Universe 1 / DMX Channel 511 = Red
Universe 1 / DMX Channel 512 = Green
Universe 2 / DMX Channel 1 = Blue
or skip over them, as in:
Universe 1 / DMX Channel 511 = Unused
Universe 1 / DMX Channel 512 = Unused
Universe 2 / DMX Channel 1 = Red
Universe 2 / DMX Channel 2 = Green
Universe 2 / DMX Channel 3 = Blue
Hopefully this better explains how controllers using standard DMX addressing handle more than 170 pixels on a single SPI output.