why's there unequal distribution of kinetic energy in water molecules?
Why do the surface molecules have more kinetic energy than the other molecules?Because that's the way it is - there is always some distribution of energies of molecules, it doesn't hold for water only. Molecules collide all the time, partially exchanging their kinetic energies. Some get faster, some get slower in the process. Total energy stays constant, but individual speeds/energies don't.
See for example https://en.wikipedia.org/wiki/Maxwell–Boltzmann_statistics
Why do the surface molecules have more kinetic energy than the other molecules?