There shouldn't be. I thought the same thing when I started doing it with my DSi, but now I'm fine.Just now a friend of mine is trying to RNG on his DSi... He can't hit the seed even after several attempts counldn't reach the right seed.
a couple of minutes later he tries with my DSlite and gets the seed right several times actually...
So the question would be: Is there any difference RNGing with a DSi?
edit: BTW his game is SoulSilver..