Moisture can be held in varying degrees of bonding. Formerly, it was considered that water in a food came into one or other of two categories, free water or bound water. This now appears to be an oversimplification and such clear demarcations are no longer considered useful. Water is held by forces whose intensity ranges from the very weak forces retaining surface moisture to very strong chemical bonds.
In drying, it is expected that the water that is loosely held will be removed most easily. Thus it would be expected that drying rates would decrease as moisture content decreases, with the remaining water being bound more and more strongly as its quantity decreases.