433MHz quarter-wave length antenna: longer is better?
Actually, it's not that strange. A quarter wave monopole antenna (l = λ/4) is dependent upon a reflective plane normal to the antenna to act as a dipole. (Like a VHF antenna on a car.) Without this plane, the quarter wave antenna will not work properly. 

The solution, as you've found, is to use a kind of half wave dipole antenna with a length equal to half the wavelength of the signal (l = λ/2). 

There is no harm in using a longer antenna. The reason for recommending a quarter wave antenna is probably that it has a higher antenna gain than the dipole, as well as the fact that it simply takes up less space... It is also simpler than a "real" dipole.

The wikipedia articles on dipoles and monopoles are quite informative.