I just finished narrating a 438-page (50 chapters) book, and the publisher has told me that 10 of the chapters sound like they were done in stereo. I never record anything in stereo, but I checked the chapters anyway. They were all recorded in mono and were playing back in mono.
Is there another problem that could make a file sound like it was recorded in stereo when it wasn’t?
If you gave them a mono mp3, then the problem must be at their end.
If you’ve created a stereo (dual-mono) mp3 of your mono recording,
that can accidentally generate a pseudo-stereo effect, depending on how it’s encoded.