I have a question on mileage calculation on United flight.

If I fly between JFK and NRT and flight is via SFO, how many miles do I get?
JFK - NRT is around 6700 miles and JFK - SFO - NRT is around 7900 miles.
I know that I get 7900 miles each way if I buy the ticket like JFK - SFO - NRT, but if I buy the JFK - NRT ticket and no non-stop flight (UA 801) is available and I can only get UA 853, which is via SFO, how does United calculate the milaege?
Do they still count as "direct" flight and give me only 6700 miles or give me 7900 miles?

I remember that when I took Korean Air from JFK to ICN (Seoul), no seat was available for non-stop flight and I took "direct" fligth via ANC. I believe they only gave me the same mileage as non-stop flight although the actual mile of direct flight via ANC was longer....

Does anyone have clear explanation??

I appreciate any information...