There is a good chance that the one from your hardware store isn't a HO, and the one from the LFS is a HO, which would account for the visual appearance of been brighter. A normal T5 only gives off about the same amount of light as a T8, whils a HO gives of considerably more.I have noticed a difference. I have two 17 watt 6500k T5's, one I got for $15 from my LFS and the other for $5 from a hardware store. The one from the LFS is much brighter then the hardware store's.
Maybe its a bad bulb. *shrug*
Of the tubes I have tried, 3 of the best are HO 865s from Osram, Philips and GE. All of these were just as good as tubes costing a lot more money, and all of them cost me about £4 (GBP), as opposed to the £10 to £15 we pay over here for aquarium makes of daylight tube.
One other thing to keep in mind, 8,000k tubes LOOK brighter than 6,500k tubes, but give off the same 'quantity' of light, this is because the 8,000k tubes give off a 'whiter' light, less yellow, so our eyes perceive them as brighter.
Hope this helps.