How do Fluorescent molecules emit light in a different wavelength than the one needed to excite them?

It is explained quite well in the corresponding Wikipedia article: https://en.wikipedia.org/wiki/Fluorescence#/media/File:Jablonski_Diagram_of_Fluorescence_Only.png The atom emmits the energy in several smaller portions falling back to the ground state passing short-lived intermediate states which have energy values between the excited and the ground state (see linked graphics).