This is because particles are assigned ID numbers, left to right, top to bottom. So the top left pixel (lowest ID) is simulated first, then the next ID number... This means that if the left pixel is near fire, it will turn to lava, then the next pixel (to its right) will be near lava, so it will change.
Okay. So its not a glitch per se, rather, its a result of how TPT does calculations. You may notice that a vertical test of this same type will have the thrm instantly ignite top to bottom, but is slower bottom to top.