Usted/ustedes - Why third person?

I often wondered why the 'usted' form of a verb is the same as the 'el' or 'ella' form, and likewise for the 'ustedes' and 'ellos' or 'ellas' forms. So the formal version of 'you' is referred to in the third person. This can be confusing to a student of Spanish because the pronoun (el, ella, usted etc.) is usually omitted, so it may be difficult to see who is doing the action. For example -

el habla = he talks
usted habla = you talk (formal, singular)
habla = he/she talks, you talk

ellas corren = they (female) run
ustedes corren = you run
corren = they/you run

This seemed a little strange to me until someone pointed out that 'usted' can be loosely translated as 'your honour'. Admittedly it wouldn't ever be translated that way in reality - the dictionary definition is 'you', but it helps to understand why the third person form is used. Imagine talking to a judge, or anyone else you might call 'your honour', compared to speaking to anyone else -

Your honour is too kind.
You are too kind
He is too kind

Your honour does well
You do well
She does well

Your honour talks
You talk
He talks

Notice that in each example the 'your honour' form is the same as the 'he/she' form, rather than the 'you' form. The 'usted' forms in Spanish work in just the same way.