I suspect you are correct mightymads about the mirroring thing being due to Doyle's laziness. Doyle can create marvellous stories and characters, but he appears to mix this in with bouts of inconsistency, because as we've noted: that Doyle didn't care for consistency and just wrote what was entertaining.

I mean, he was successful, but a tad more consistency would have been nice ;)

Ooo thank you for checking! ebooks can be so useful for such things :) I wonder why Doyle didn't use one of the three more familiar inspectors?