A young plant produces lower pitchers for many reasons. The larger size often associated with lowers help anchor the plants to support the young vine in reaching towards higher points. The larger ground pitchers also more attractive is used to capture larger prey, prey associated in insects that walk through the understory (like roaches, beetles, crickets, etc.). This would explain the more vivid coloration encountered in lower pitchers. Lower pitchers tend to be produced usually under the protection of its leaf blade (to keep rain water out of traps) with attachment to the front of the pitcher. This facilitates the trap to always face under their respective leaf blade. The lids are usually larger almost to cover the opening from rain and debris from falling in. The lowers also produces wings (like fins) and its reason is not for certain understood fully. Many hypotheses vary from being ladders to the mouths, to create insect neon attractant (by use of UV light) to guide insects to the mouths, to adornment for added attraction.
The upper pitchers tend to be less colorful and somewhat different in appearance. Not all species have dimorphic (two-form) pitchers. This dimorphism may be due to several reasons. Mostly I feel that a less colorful upper (where uppers are usually higher up in the canopy) pitcher would not attract predator animals from destroying pitchers out of curiosity or food. A colorful trap would attract birds and monkeys which would destroy traps thinking they were fruit ripening. While many flying insects which do not see color, see UV light and many green pitchers have a high UV light attractant pattern. Upper pitchers tend to be smaller also lightweight will help the vine climb higher without heavy weight bringing them down. While upper pitchers tend to have a more slender lid or even lid reflexed, this would allow rain and other substances to be allowed entry. Upper pitchers have tendrils that are attached from behind to help keep pitchers facing outward away from the vine. A twining vine also helps support these pitchers as it clambers into the canopy. Upper pitchers tends to feed on smaller insects such as ants, some flying insects and possibly other substances as hypothesized for N. lowii.
Michael