From the learn-something-new-every-day desk:
I have been flying for decades and this just never happened before, so I was like ....
Had an expiring DL voucher and completed the ANC-SEA-AMS leg of an FAI-DUB route on DL #142 .
DL #142 was the flight number for ANC-SEA, then there was a 5 hour layover, aircraft change, and 142 was also the # for SEA-AMS.
The miles posted a couple of days ago and it showed up as ANC-AMS. I wasn't amused so I emailed AS. They fairly promptly replied:
I have reviewed your Mileage Plan account and the mileage credit to your account did post correctly. For that flight #142 on Delta Airlines, it is considered a through flight which means that it has the same flight number all the way through and for through flights, the mileage credit is given based on the departing city to the arrival city. Unfortunately, they do not earn extra mileage or an extra segment for a layover on a through flight.
There you have it. Depending on how one looks at it it's 40% fewer miles credited. Bummer...