Why would traveling along the z axis cause you to find more nether fortresses?
Amidst doesn't show any pattern to the locations of Nether Fortresses.
BTW, Amidst maps Nether Fortresses in the Overworld where you would have to place the portal to get to that fortress.
If you click on the fortress icon Amidst will display the Nether coordinates of the fortress.
If you look at a seed zoomed out in amidst and only show nether fortresses you'll see that they are in a pattern of strings oriented north-south on the z axis. So once you find a fortress its a good bet traveling north or south will find you more in that string unless you've found an odd one that is way out in the dead zone between strips. Here is a pic I found online showing what I'm talking about:
Well, from 2012 to 2017, this thread is one of the best I could find regarding Nether Fortresses, good call answering the question over 5 years, reasons I like forums and threads like this one... always usefull. Thanks