I did some checking. The Delta flights are with Korean Airlines, which is also offering those days at $1120. The difference is that with Korean, you would have to make two stopovers(Taiwan and Seoul) on the way back. Their one-stop package is $1510. Incidentally, the $1100 Delta tickets(Delta Flight 7854) and the $1510 Korean airlines tickets are both for economy seats on on the same planes, Korean Airlines Flight 82 and 86(KE82 & KE 86). Same seats, different price!
So, the bottom line is that only Korean airlines has flights leaving on 12/20 and coming back on 1/5 at those prices. They have two flights that day, one is the red-eye leaving at 12:50 AM, in other words, very late on Monday night, but technically Tuesday. The other leaves at 12:30 PM on Tuesday.
Delta has the market cornered on the cheap Korean tickets. When those two flights fill up, no more $1100 tickets.
When the $1100 one-stopover flights have filled up, there may still be tickets for $1100 for a 2-stopover return flight through Anchorage, Alaska, but do you really want to have to go that route?
There is the possibility of other airlines with pricier seats, that remain unsold, offering them for less at some time as Decmeber nears, but it is unikely that the price will get as low as $1100.
It is almost certainly a case of buy now or pay more later. Korean has always been the cheapest airline. The only difference is that Delta has bought up large blocks of their seats.
JayBee
P.S. The only way a travel agent could beat these prices is if he is affiliated with a company that has already bought large blocks of DEC tickets to LOS at a discount. But even if that were the case, that company would be watching the market for prices on DEC flights to LOS, and be very unlikely to want to undercut the market. What would be in it for them? They know the value of the seats.