What is the explanation for the smaller number of tRNA than codons?
Th reason for this is that for the third base of the tRNA non-Watson-Crick pairing is allowed. This phenomenon is called "Wobble base pairing". See the figure (from here) for illustration (from here):



If you have a look at the codon table for amino acids, than the variation in the code for one amino acid mostly happens on the third position (from here):



This allows a smaller number of tRNA than 64, as there is some flexibility. The third base can be represented like this (taken from this answer):



In the table Cricks predictions are compared to the pairings found in experiments.