As
Moriens says above, it is likely that you saw an R - Q fare combination.
There is no published R fare that's lower than approx. $1300 inc tax:
[KVS Availability Tool 2.8.0/Platinum - Sabre: Fares/ZUJ/SG-USD]
Code:
SEA Seattle/Tacoma Int'l WA US [KSEA]
BJX Leon/Guanajuato Del Bajio MX
RT Mar-Feb
Carrier From To Fare Cur Expiry Min Max Fare Basis
--------- ------ ---- --------- ---- --------- ---- ---- ----------
CO SEA BJX 1164 USD RA7N
CO SEA BJX 1264 USD RA0N
CO SEA BJX 1281 USD RR30N
CO SEA BJX 2348 USD C
CO SEA BJX 2348 USD CHOLNRO
CO SEA BJX 2348 USD COX
The full Y fare is close to $2300 so that's unlikely to be a good option in this case either.