There are some parts of cars that don't require all the screws (or bolts) to do the job they were intended to do---similar to how commercial jets are capable of flying with only 1 or 2 engines rather than the all of the ones actually on the plane....
I'm going to go with "not having enough screws"---based on experiences rather than simple logic.
A screw left over... You've forgotten to fit it where it should go.
A screw left over that appears not to match any others you had removed and refitted... the ill-begotten fruit of cluttering your toolbox with all manner of obscure fasteners, brackets, clips and general small rammel acquired over many years.
A screw missing... It is either simply on the ground below the car or in a harmless bodywork recess.... Or it has found somewhere inaccessible whence it can cause as much havoc as it likes.
Your question reminded me of me when I first started swapping engines in my cars many moons ago. The very first time I performed an engine swap, I ended up with around six or seven bolts left over. I scratched my head for a while, but was stumped as to where they went. So the next thing I did was take the car for a test drive - It performed like it should, with no rattles or funny noises, so I left it the way it was, and tossed the bolts.
Bozette 