So SG minus FG gives you the initial drop, e.g. 100 point drop, you then add say honey, that you add and if you made sure it was well mixed then took a reading and it had cranked back up 20 points, if the yeast used had enough ability/tolerance left then its quite feasible that it could either continue or restart fermentation back down to the original FG.
Then it would have had a total drop of 120 points, which converts to a higher level of alcohol.
Its why you need to think about not starting a batch too high, but also why you need to be aware of the yeast tolerance and that if you ferment dry with the intention of back sweetening, you need to either stabilise befor adding back sweetening sugars or use non-fermentable sweeteners.
It does get harder to work out if you add fruit whole or frozen. As you might get a sugar content reading from a refractometer but it still wouldn't give you a reading of how much sugar has been added.
It can be worked out but its very technical to do (calculate sugar percentage, then how much weight of sugar then how that converts into gravity numbers, etc etc).
So personally I just ferment dry then flavour with fruit and steep it in the mead to attain colour and flavour, then back sweeten to taste.