There are Metric fine threads...
Sticking modern metric threads against old imperial ones - is that not more likely to be something to do with the materials than the thread form?
The most awkward ones I've found were on the engine of a vintage Morris lorry (same one as in the Bullnose). The bolts had imperial AF heads but metric threads.
Apparently Morris bought the engine from Hotchkiss, who being French used metric threads. To allow British mechanics to be able to work on the engines and to avoid redesign work on the engine Morris just changed the bolt heads. Just the sort of thing people would do nowadays to ensure control of the spares market!
Don't look so embarrassed, it's a family trait...