Where did the bucket come from? Was it a beer bucket- those can be smaller than a wine 7.9 gallon primary. If you are sure it was only 5 gallons you made, then the dilution ratio was wrong for the whole batch. That throws off the intended ABV, acid levels , body etc. If you were supposed to add lets say 2 gallons of water and you only put in one gallon, the chemistry was wrong when you made it. After being certain of the values of water added, fill level, volume of bucket etc - I would add the extra gallon of water. I know most of you will think this is wrong, but if it was supposed to be in there, it should still be in there. As long as the fermentation completed, it shouldn't alter the outcome of the kit.
You could always call Winexpert and ask their experts for their opinion. If you do, let us know the results.