G'day viewers. Riddle me this please!

I purchased an I Class American Airlines ticket from PHX to SEA. 1,106 miles distance.
Normally I would earn 100% base miles + 50% Class of Service Miles (combined 150% EQM) + 125% Elite Bonus Miles. RDM=(1,106+553+1,383=3,042) and EQM=(1,106+553=1,659).

HOWEVER, on my last flight I chose to take an earlier same day flight. I chose an economy seat and gave up my paid first class seat. This is not something that I would normally do but it did save me 5 hours and thus made sense at the time. Especially considering the non-existent first class service on American Airlines right now. To my surprise, I did not accrue mileage according to my original ticketed class. I only received miles for an economy ticket. Is this correct?

I got:
100% base miles + 0% Class of Service Miles (combined 100% EQM) + 125% Elite Bonus Miles. RDM=(1,106+1,383=2,489) and EQM=1,106.

Should I get 3,042 miles or 2,489 miles on a paid I Class ticket flown in economy? I was always under the impression that miles were earned according to ticketed class and not class flown.