"I asks"? How is that?

The Adventures of Sherlock Holmes — The Red-Headed League:

“‘I wish to the Lord, Mr. Wilson, that I was a red-headed man.’

“‘Why that?’ I asks.

Why has the writer written "I asks" and not "I ask"? It is definitely not a mistake, for I've encountered this a handful of times afore in an array of different books.

Here's a theory from a source:

That is non-standard English, which is only spoken in a specific part of Britain.

Correspondingly, perhaps the author was attempting to cause the reader to understand this about the character.

Is this true and is this indeed the reason?

Is this true and is this indeed the reason?

Vincent Spaulding, the pawnbroker's assistant, is a dialect speaker, Cockney very probably.

To reflect how that character talks. For whatever reason, that's how that character talks. Maybe it's his dialect. Maybe he lacks education. Maybe it's both those things. Maybe it's neither of those things. Maybe it's something else entirely. There are lots of maybes. Rest assured, whatever that maybe actually is, it becomes part of Sherlock Holmes' analysis, because Sherlock Holmes misses nothing.