I think this is a key point, and one that really is missed by many lifters. We all have a certain "rep count" that our bodies respond to best. For me it seems to be largely singles and some doubles. For someone like Rob (a raw lifter with a body shape somewhat similar to mine) it seems to be doubles and triples. For my wife (an equipt lifter, much smaller than we are) it is 3-5 reps. If she can do a single, she can almost always double it, where for me i can grind out a maximum single and that second rep may take an act of God to happen.
In the end, maximizing the rep scheme that works best for you will lead to the best strength gains with a particular exercise. You just have to figure out what the heck it is and test it periodically to make it work.
Your wife can do that because she's a woman and her fiber make up is diferent than men. All women are like that. We have countless female athletes that will hit 135x5 and then you add 5lbs and it smashes them. This is very common with women.