You said you had some left in the pot after you poured off 5 gallons. Doesn’t take that much to dilute the wort. Either that or you read the hydrometer wrong. Extract is pretty consistent but I suppose it could be off but I doubt it.
Another variable is did you get all the extract into the pot?
83 degrees is pretty warm, which as Brew Cat said is why you had an explosive fermentation. What is the actual temperature of the beer and how are you measuring that? If you’re relying on ambient temperature in the hallway, then you’re beer is easily 5 degrees warmer than that. That being said, chances are pretty good you’ll have some off-flavors due to the warm fermentation.
I’d recommend reading up a bit on temperature control. You may want to consider the “swamp cooler” method to keep the carboy cooler during fermentation. Slap a stick on thermometer to the side of the carboy and use that, as it’s a lot closer to what temperature you should be reading.
Also, if your ground water isn’t cold enough to cool your wort to at or just below the recommended range for your yeast then you need to consider an ice bath or another cooling method. Fermentation temperatures and yeast management are probably the most important thing besides sanitation.
