Not defending QF here or anything but my experience is that infant fares tend to be a flat 10% of the adult fare (obviously base fare) and that probably explains that. While 3K may charge a set amount, you're looking at a fare offered by QF which includes 3K, and that will have different conditions. Remember there can be many fares in the same market, even for a simple sector like SIN-BKK, and even for the same flight even equivalently priced fares MAY have different conditions depending on the carrier that issues the fares.
Also note some one way special fares are not always reflected in return fares.
I am not saying that applies to your specific example John, and I don't know why the websites would show different things, but remember if you booked after your wife did then availability could well have changed and heck fare offerings could also have changed even based on things like AP rules, or just what is offered at any particular point in time.
I'm more trying to find a possible explanation rather than suggest there isn't a problem with how QF displays some fares, specially connecting ones.