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[NFossil]

(Hell yeah, someone besides me giving the correct theory of pressure. High fives for Quietust.)

So here's the thing about magma pipes. The "magma appears from the top and rains down" explanation is a wacky myth. Maybe it was true in an earlier version or something; I don't know how else it would have gotten so popular.

Basically, the magma flow tiles at the bottom of pipes find a path to the lowest empty space in a vertical line to create globs of 7/7 magma.
I think gtmattz basically has the idea, if when he says "top level" you understand he means the current top of the magma in the pipe, not the absolute top of the pipe itself. If you've drained all but 5 levels of your pipe, the new magma will appear on the sixth level and then fall into any empty space on the fifth. So there's this nice bubbling action in a refilling magma pipe. This is a fine opportunity to build a platform in the middle of your partly-drained pipe and chain elven virgins and kittens on it.
Note also that if you channel into certain places in the pipe for a pump intake, a magma flow tile can be aligned to put magma up through that pump intake channel when you wouldn't normally expect magma to flow upward like that. I'll leave the details of how I discovered this to the reader's imagination.

I'm sure refilling magma can appear 2 or more levels above the current level of the pool. So if you drain it to 5, new magma can appear from 6 to maybe 8. However, magma filling from the bottom is still true in a way. New magma never appears above floors.

[Kanddak]

I'm sure refilling magma can appear 2 or more levels above the current level of the pool. So if you drain it to 5, new magma can appear from 6 to maybe 8. However, magma filling from the bottom is still true in a way. New magma never appears above floors.

I've drained a magma pipe and then stared at it for a while and never saw anything like that happen.
One thing I did notice is that if, say, level 5 is full of 4/7 and 5/7 magma, it will still spawn full 7/7 blobs on level 6, which then fall down into the available space on level 5.
Is it possible that in whatever instances you observed, the pipe happened to generate magma on the same tile two times in rapid succession, so that there was still a bit of magma on level 6 allowing the new magma to appear on level 7?

I can imagine an experiment to test this.
I've confirmed that magma creation pathing will go up through grates or horizontal bars.
Build pumps to keep a magma pipe drained down to a given level, which we'll call level 5 because we've been using that example so far. On level 7, built alternating rows of floors and grates over the magma pipe. Then fill the level with cats. If the cats die, magma must be appearing on or above level 7. If the fort runs for a long time without burning any cats, then magma appearing two or more levels above the current magma level in the pipe is an uncommon event at best.