Loss on drying measures all weight loss after heating to a specific temperature for a specific time. This value will include moisture as well as any volatile components or solvents that may be present. Moisture content is only a measure of the water present.
Often the two values are identical, if water is the only component that is lost, but if you are measuring loss on drying, you can not verify how much of the loss is caused by water.
That's what I known about the water content and loss on drying, but I can't found any authoritative documentation to tell may customer.