It's like anything else. You play cost, quality, and necessity off one another and make your buting decision. A person who needs resin right now can't wait for a gallon to be delivered. Also, a person who just wants to buy a little to try out casting is less likely to mind paying a premium.
This is little different than how I shop at woodcraft. There are very few things that I always buy from them because their prices cannot compare to buying online. However, sometimes you need a kit, a special blank, or certain supply right now and can't wait for an online order to arrive.