I believe it is per WLED install max of 510. If only 480 are being controlled and and only 480 are installed / configured in WLED, it should match up just fine.
Five years later, I had the same issue today and you helped me fix it! I am using RGBW WS2814 strips, so 4 channels per pixel. 510 is not divisible by 4, so I changed the universe size to 508 (512 could also work as the maximum?). And there you go — no more weird colour changes in different universes! Thank you guys!