It makes perfect sense if he still has the stock hammer. Tuning the DA pull requires a completely different approach than tuning the SA pull; a light SA is almost completely dependent on the length and angle of the hammer hooks. The SA shot breaks when the sear slips off the hammer hooks, but the DA shot breaks when the disco slips off the trigger bar so they're two separate mechanisms.The hammer spring and trigger return springs have a huge effect on the DA pull weight, have little effect on the SA pull, the SA pull is all about the hammer.
Thank you! That is awesome information to know.