For anyone interested in why this happens, I posted a theory on a different forum. Here it is, copied.
When Lucas jumps the second time in air, you notice that it's sort of spacey. There's this weird gravity thing going on. What we figured is that the jump is composed of two forces. One is the up force that causes Lucas to jump. The other is the down force which causes the up force to decay. The end result is that parabolic, spacey movement Lucas has. For Lucas's PK Fire, we think there's a slight movementary modification in down momentum. It's really, really subtle, almost nonexistant. But it helps that he juts back when he uses his PK Fire. So what we feel is going on with his Zap Jump is that by pressing Jump and PK Fire at the exact same time, you're actually canceling out the down vector of the air jump, causing Lucas to inherit all of the momentum of the up vector. It results in him flying 3U into the air at high speed. And he still ends up shooting a PK Fire before the apex of the jump.