ILLUSTRATOR EXPORTING RANDOM PIXEL WIDTH TO PNG

Dear all, I am absolutely gobsmacked with the random behavior of  Illustrator. Dre a series of squares all with black fill, 5 px stroke, and different colours. When exported to a PNG the 5 px stroke comes out with random width. Pictures attached for a better understanding. What on earth is going on? Has anyone seen this ever?

The procedure is as simple as exporting the file to a PNG without aliasing.

Any ideas?????