The guns are randomly generated by the system. This means that there are possibilities of up to a million different guns(you didn't even read the wikipedia article yet linked to it, what the forget.)
"The game will also have a unique feature similar to Diablo's random level generation system. This new system will generate more than 1,000,000,000 different guns as well as at least 2,600,000,000 different item and armor variations"
Hence the quote that you put on is now in context:
"We saw a ridiculous amount of guns, but perhaps the strangest was a revolver that fired shotgun shells. Gearbox is constantly surprised with what the system comes up with. They've seen rifles shoot everything from homing darts to rockets. 'One of the guns tracks onto something and locks, and after three seconds, [the target] suddenly explodes,' director Matthew Armstrong says."