It's not about typos or mishearing stuff, it's about words being jumbled in memory. Unlike a sequence of digits, people don't store words "engineering" in their head as a string (eg. "E-N-G-I-N-E-E-R-I-N-G"). It's stored as something like "[concept of engineer] + [present participle]". That's far more likely to get jumbled in people's head during recall.