There are at least two sources that influence patterns of behavior: the individual that is doing the biting, and the individual that is being bitten.
The example of lice in one of the comments is a good example. Three categories of lice that get on humans are: head lice, body lice, and pubic lice. They are related species (possibly the same species in some cases - https://www.insidescience.org/content/are-head-and-body-lice-same-species/550), but each type has their own preference for habitat and feeding. There are all kinds of reasons for a species to have a preference, and some stick to those preferences quite strongly.
As for your mosquitoes, their absolute preferences are still not known but blood seeking insects do have their senses attuned to clues such as carbon dioxide and heat (that help identify potential hosts) as well smells and other influences on their senses. There might be something special about the side of your right knee as far as mosquito senses go, but not every individual in a species sees, smells, hears, etc. the same way. My brother has a gene that makes broccoli taste very bitter, but I am fortunate to not have those taste buds. Color blindness is another example of differences in perception, but even without being color blind, what you see as red is probably not how someone else sees red due to natural variations in the patterns of sensory cells in the eye. For a consistent pattern of behavior from the mosquitoes, in spite of individual differences we still need to consider another source of variation that might lead to this mosquito preference: the host.
If the variation of individual mosquitoes is trumped by a consistent pattern of predation, perhaps the mediating factor is the host. Your knee could have a special smell, or other sensory flag that attracts mosquitoes. There are efforts to understand how a mosquito's perception directs who gets bitten: http://www.scientificamerican.com/video/future-repellents-mess-with-mosquit2013-12-23/ . Not enough is known yet about the preferences of mosquitoes to say for sure what makes your knee so good. There is also a chance that your knee is not special to the mosquito because it appears the most appetizing.
Host behavior is another possible influence. The knee is just far enough away from the reach of your hands. Conscious or absentminded grooming would both be limited at that distance. The knee may also be better than an ankle or a foot, because of what is worn or because the knee moves less than the lower leg during walking and other movements. If you are bitten on the lateral (outside) side of your knee, this would add evidence to this hypothesis of your movement influencing mosquito behavior, because the inside of your knee more frequently brushes against the other leg. The frequency of the inside of the knee being a safe place for a meal is much lower.
As for why it's the right knee rather than the left, there is possibility that that is also a reflection of you, rather than the pests. Humans display preferences in dominance for both hands and feet. One side of your body may be selected more often than another because of you being right- or left-handed, or right- or left-footed. This could affect grooming habits (how you swat at the bugs), how your cross your legs, which side you carry items or lean against other objects which might brush against the body, influence how well you spray yourself with bug repellent or who knows what else. (Just for curiosity's sake, ¿are you right-handed or are you left-handed?)
In short: There's a lot of evidence to collect before we could come to a definitive answer. There are a lot of possibilities, including that your knee may not be the tastiest spot, but rather a relatively safe spot for dinner.
For more unempirical thoughts on the relationship of insects to the lower limbs, please see: http://fishboyridesagain.blogspot.com/2005/11/bugs-and-trousers.html
