Very simply, all sencha has L-theanine which creates the umami flavor and catechin which creates astringency.
Shading reduces the catechin in the leaf, so that it doesn't overwhelm theanine, and allows a sweet, savory tea to be steeped from the leaf. Gyokuro is shaded for maybe 3 weeks, but also is created with other techniques including fertilization and concentrating nutrients by plucking off leaf buds.
Kabusecha is shaded for shorter, and used to be generally used to increase the theanine content of sencha...a blending technique. Blenders blend a machine-harvested kabusecha or even gyokuro leaf with mid-grade sencha to increase the sencha's overall umami flavor, making it a "better" tea.
Different cultivars of plants also naturally produce different levels of the above components too.