Superlatives of adjectives ending in -lis are usually formed with the suffix -limus. For example, the superlative of facilis is facillimus. So, why is the superlative of fertilis fertilissimus, rather than fertillimus?
The superlative ending -illimus is actually exceptional, although the exceptional adjectives are quite common. The adjectives with this superlative ending are facilis, difficilis, similis, dissimilis, gracilis, humilis. Other adjectives ending in -ilis, including fertilis, have the more regular superlative -ilissimus.
See §126 in A&G.