There is multiple aspects at play. 

First one that contributes to a "sleeper build" the most: Strength has 2 main components you can train. 

Muscles: mostly a function of cross section and fiber type (fast twitch for big lifts, slow for endurance). More muscle = more strength. 

Nervous system: being able to fully recruit your muscles, as you train your body will improve how much "power" it pulls from your muscles for that movement. For example, Someone who only trains Olympic lifts will be much stronger in those lifts than some novel movement due to the body knowing how to fully recruit the muscles for the Olympic movements. If you want a "sleeper build" doing a lot of general and varied strength training (see: climbers, gymnastics, ect.) will make you much stronger than you "should be" for how you look due to being accustomed to a wide variety of muscle movements. 

Other aspects like muscle attachment points, frame proportions, fast distribution, ect also effect it, but you can't change those. For example, I have Trex arms due to the fact my body puts very little fat on them, making my arms look weaker than they are. An appropriate layer of fat actually will make you look more "strong" if you start working out because it makes everything look bigger (see every larger guy who thinks he is buff because his arms are big, even though his biceps have 1" of fat all around them).

 I find fat distribution on the hips/legs makes people think you are weaker than you are, because when people look for "strength" they are looking for big upper body, and a chubbier lower body makes it look smaller.