Plainsman Clays, for example, publishes dry and fired shrinkage data for their clay bodies. The former is the shrinkage from wet-to-dry. The latter is the shrinkage from dry-to-fired. You cannot add the dry and fired numbers together to get the total because the two are based on different starting points. Consider this example: 6.25 dry shrinkage + 6.66 fired = 12.9 whereas the actual total shrinkage is 12.5%. Shown is the way to calculate the total shrinkage correctly if you only have drying and fired values (thanks to Tom Hittie for deriving this for us). Of course no one is going to bother actually doing this calculation! So just remember that the actual total is a little less than adding the two together.
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
Clays used in ceramics shrink when they dry because of particle packing that occurs as inter-particle water evaporates. Excessive or uneven shrinkage causes cracks.