The rise of water levels is way more complicated than just taking the inches of rain and converting that to gallons of water. The watershed for Lake Martin is about 3,000 square miles in size. Rainfall at the lake itself has very little impact on water levels. Precipitation over the entire watershed, most of which never makes it to Lake Martin, is the key to the water rise. Some infiltrates, some evaporates, some is collected by stormwater basins, some gets into surface water bodies that discharge into the Tallapoosa, etc. Keep in mind that during its early days of operation, they used to drop Lake Martin 50 feet in the winter and it would fill up every spring. Its about managing discharges from the dams on the river in light of the precipitation in the watershed.
The fact is we had a tremendous amount of rainfall in the Tallapoosa River watershed and even if the level were at 483' it would still be 1 foot below full pool. The dams on the Tallapoosa did exactly what you would expect, the containment of flow to reduce flooding. This was actually a pretty good example of why having winter pool at 483' is not a problem.