I really don't know, but If I had to guess I'd say it has to do with sales volume, it's simply cheaper to manufacture the tank that sells the most.
Very interesting question though.
dave
I'll hazard a guess, and say that Dave's right - I'm willing to bet that the standard 10g is one of their top selling sizes, and since they sell more, they cost less to produce.
I work at a pet store and the above are correct. The reason all-glass 10 gallon aquariums are cheaper than 5 1/2 gallons is simply because we can sell more 10's. For every one 5 and a half we sell probably six or seven 10's.
I work at petco partime. We got in aquariums this last load. we had 5 or 6 5.5 gallons, and two whole pallets of 10gals stacked about 8 feet high. we've already sold half a palet of 10's and no 5.5's to my knoledge. So yeah, it comes downt to mass production vs. specilized fabrication.