With a Smartguage, the gauge indicates what is left in the battery as a percentage. The gauge neither knows nor indicates the capacity (total Ah) of the battery. The gauge works out the state of the battery solely from monitoring the battery terminal voltage over time.
I find that when charging has finished and the battery starts to be "used", the gauge stays at 100% for some time and only then starts to drop.
Regarding the OP, if the gauge has indicated 100% for some time during charging, then the battery holds as much as it can and I would expect to behave as I have indicated above.
Note if your battery bank is old/degraded/sulphated the capacity (the useable Ah) will be far far less than when the batteries were new. The gauge will still go to 100% when the battery has absorbed as much charge as it can, but will drop more quickly under discharge as the capacity of the bank is reduced.