Subtle Lack of Randomness in Maze Generation Algorithm in C

This concerns what I can only surmise is a flaw in somebody's code used to generate random mazes. The code is somewhat long, but most of it is commented-out options and/or not concerned specifically with the randomization.

I got the 2001x2001 maze from the link dllu put up and saved as a png here. From that, I have created this. To get the blue pattern, I started filling in dead ends starting in the bottom left-hand corner of the maze. According to the backtracker algorithm he used, that's the point at which the maze begins to be generated: so if you following the trail of dead ends created by that, you can systematically fill in all the dead ends on that side of the maze. In other words, the central blue mass represents the total accessible area starting from the bottom left, up to the sole frontier pixel at 2678 x 1086.

But there's something immediately anomalous, in that the blue "fractal" seems to repeat itself. Indeed, by overlaying one part of the fractal, rotated and mirrored, you can see there is an exact correspondence of shape. Another anomaly from this overlay maps part of one of the continents onto another, but strangely only a sliver of the landmass this time. Evidently these aren't the only auto-correspondences.

But besides the shape of the dead-end components, there is repetition of the actual pattern of the walls when you zoom in. Strangest of all, the repetition isn't exact, but only maybe 50-60% of the walls correspond. Zoomed and constrasted sample of a region:

Long question short, what in the code is causing this obscure lack of randomness?