If Ness's level up, RNG advance after battle. But that RNG is NOT constant. I don't know why it is not constant.
When I defeat Ness's Nightmare in my Any% speedrun, I think those pattern are about 10.
EarthBound Any% looks constant RNG after Ness's Nightmare battle.
How do you get constant RNG?

Mother 2 has a little bit of extra randomization that doesn't occur in EarthBound. When I looked at it briefly in the past, it appeared that HP increases from Vitality updates were the cause. It seems that how quickly you advance that specific text box affects how many RNG updates occur at that time. You could try delaying long enough to see if there is some upper bound to it.

Thank for answer.
Hmm, it may be difficult to manipulate RNG after Ness's Nightmare...

I did a little bit of digging and I'm not 100% certain, but it appears that the scrolling HP you get during a vitality update causes RNG to advance a few extra times. During my testing I tried to let the HP scroll all the way up after you get the HP update during the level up and got a consistent RNG after the battle. I went from 31 hp to 43 and it stops the scroll until I advance the text again, then goes to 45 for level 3. Maybe this could help you figure out whats going on.