There could be a couple of explanations for this (maybe more?) but this is what I've seen in my experience as an ultrarunner.
1. If you are walking / running properly, the centrifugal force generated by swinging your arms will gradually lead to edema in the hands. I see this to a minor extent in most every 100-miler I attempt -- the problem is later in the run, and usually during the cooler part of the day when I'm not sweating as much.
2. Electrolyte *imbalance*, mainly either too much or too little sodium. For most people, I would suspect the problem lies with too little salt, but for those using buffered electrolyte caps or other salt tablets, I've seen sodium overload occur (end up peeing a lot and you still can't get out all the water).
If you are not taking in adequate electrolytes, the imbalance in salt levels between the blood stream, the cells, and the extracellular spaces essentially results in a trapping of the water in the tissues as the sodium is lost in sweat. The same thing can happen if you are taking in too much salt. For those interested, look up "sodium-potassium pump" in a textbook of physiology to get you started.