Rolling Stone Magazine has run an online version of a Kurt Cobain interview from their January 27 1994 issue. In it, Cobain breaks down Nirvana's songwriting shares. I was wrong about him receiving all the royalties. If my math is correct, it appears he took a total of 87.5% while the other two band members shared the remaining 12.5%, for a total of 6.25% each. Here's how he put it:

Haven't there been any issues where there was at least heated discussion? Yeah, the songwriting royalties. I get all the lyrics. The music, I get 75 percent, and they get the rest. I think that's fair. But at the time, I was on drugs when that came up. And so they thought that I might start asking for more things. They were afraid that I was going to go out of my mind and start putting them on salary, stuff like that. But even then we didn't yell at each other. And we split everything else evenly.