Most likely yes, it has stuff besides water. Although I wouldn't lay my hand in the fire that it has it every single time.
The first scenario is the bubbling from starchy water, as Chris H already explained. This is not ruled out by "the assumption that the food has not made contact with the lid", since here it is the cooking water that makes contact with the lid, not the food pieces, and while it doesn't happen every time, you cannot tell that it hasn't happened when you lift the lid.
The second scenario is the steamed vegetables. The first suspect would be matter on the vegetable skins. Even if you are very thorough in washing, you probably won't remove all traces of whatever clings to them, be it dirt or the vegetables' own waxes. But even without this stuff on the outside, plant matter is perfectly distillable, that's how we get essential oils and many alcoholic beverages. Even if you steam whole vegetables with intact skin, the cooking process itself is sufficient to damage the cell walls and stuff starts coming in contact with the steaming water. Some of it is part of the water that condenses and drops down to the bottom (have you ever noticed that the water is green after steaming spinach or broccoli?) and some of it is dissolved into the water that becomes steam and starts clinging to the lid.
Even if you cannot see any residue visually (which is a rare case), there is probably stuff deposited there that is not water.