>I'm just curious about the provenance of this mistake

Because of English pronunciation (pronounciation? :-P). English is extremely irregular, there are a thousand of footguns in the language - both spoken and written -, so as non-native speakers we tend to make small mistakes that stick to our brains like glue, and it's very hard to get rid of (rid off? :-P).

For me it kinda makes sense to say "dwelve" because it reminds me of "dwarfs" (dwarves? :-P) that live underground!