I'm guessing at what the instructor meant...
With a DA/SA pistol, there is a long heavy trigger pull for the first shot. If the first shot is justified, no problem.
However...the second shot is SA, and only takes a light trigger pull to fire. If a DA says you did not "need" to fire the second shot, but you did anyway because of being accustomed to the first, long, heavy trigger pull...you could end up in trouble.
It is much easier to "accidentally" fire a second shot with a DA/SA than it is with a DAO or SA, where the trigger is the same with every shot. This transition from DA to SA is why I personally despise the DA/SA trigger. Under stress, it is just too easy to fire a second shot when you may not have meant to.
I'm guessing that may be what the instructor was getting at (?).