Well actually the PC DOS version also can only handle 80, it's mainly a performance thing I guess.
But it seems that the Mac is an interesting case where it's 3/4-way faithful. There are some notable differences here and there, but overall it's far more similar to the Amiga/PC/etc. versions than say the NES.
I can't help but speculate that on the Mac, the vertical resolution might be lower or something, so that it's not exactly 160 pixels. Which could potentially explain things like:
1) basher can bash without stopping after digging down 5 pixels on flat ground. On PC/Amiga/etc., you need to dig down 6 pixels instead.
2) Tricky 13 ("Ozone Friendly Lemmings") requires 4 bombers to solve on the Mac, but 3 is sufficient on the PC.
3) Mayhem 26: one of those "roots" protruding from ground can be walked through on PC/Amiga/etc., but can't on the Mac, causing the level to be much harder than intended on the Mac.
4) This is purely visual, but apparently, on the skills toolbar, single-digit numbers like 3 are displayed as " 3" (blank-3) rather than "03".
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This is why I'm getting more and more intrigued about the Mac version. Too bad I don't think there's a Mac emulator out there at this point, at least not for the PC.