I'm not sure exactly what causes the difference, but in DOS, a miner from that starting area will never have exactly 1px of solid terrain below him
It's pretty simple, in DOS Lemmings (and many other ports aside from SNES's unique mining behavior), each mining stroke (including the first) takes out exactly 2 rows of pixels below ground level, so if you need to end up with exactly 1 row of pixels, the thickness needs to be an odd number of pixels. In some cases you can use a builder to started mining higher by one pixel to create the desired outcome.
I haven't studied SNES miner in detail, but I have observed that the first stroke only takes out 1 row of pixel on ground rather than the usual 2. Subsequent strokes do take out 2. So the parity is flipped and I can see on SNES you would need even-pixels thickness to end up with exactly 1 row of pixels.
[aside: due to various factors including one of my laptops temporarily out of commission, I'll be lighter in all activities Lemmings over next few days]