If you’re using a mac in a campus lab environment or off some official image built by your employer, chances are fair to good that you’re using stuffit to decompress files, a legacy of stuffit’s heyday when it was actually a decent tool. My impression is mac folk are simply acculturated to using it even though it’s failed to keep up with the times. Consider Unarchiver as an alternative. It’s free and open source, it handles pretty much every file format under the sun, and it’s not evil. What more could one ask of afile decompression utility.
As an aside, I’d love to slowly throttle whoever manages the stuffit website with al dente cooked egg noodles. Ban it from your system. It had its day in the sun and now it’s time for it to go.