I'd pretty much have to go with genetics as being the main cause.
How much water you hold under your skin can be manipulated by a couple of factors, but ultimately its how your genetics favor you, some people will just naturally have less water retention than others..
If you're not adequately hydrated then you're going to hold onto more water simply because your body senses the lack of adequate hydration. Sodium and potassium also come into play with this as well as carbohydrate intake. There's a great article on T-Nation if you search around about manipulating subcutaneous fluid levels for aesthetic reasons.
It's quite the ***** too... I always had to deal with my water weight during wrestling because it was the easiest thing to both gain and lose.