glands only on the backside wtf?

my friend got some sativa that is really potent, nice buzz.. but on the tops of the leaves there is no resin. on the bottom, however, is a thick coat of glands. does anyone know much about this phenotype? the buds were glandy too. just not the leaf tops.

i took a pic but it sucks, you can see details. its just a pic of a cola.