Two likely possibilities, a bad connection to the battery..which could intermittently cause the problem your experiencing, or a short in the battery that is intermittent...
When you take the battery to be tested it does not go into default so it checks out OK, only to leave you sitting at a later time... I would remove and clean the battery cables and the battery post/terminals and reattach tightly care full not to strip or stress a battery post. Use a battery brush tool to clean all the surfaces, then re-attach the battery cables.
Also it is possible that the connection to the starter is also loose or something, save this for last as this will be the hardest to do.
If your battery is more then 3 years old, it's time for a new one anyway, three years is the expected life span of a battery and in a hot climate like florida for instance, it can be as little as two years... So if your battery is over 3 years, replace it anyway... The place that you took your battery to be tested, may not have done a thou rough test and or they may not want to replace a battery that is still under warranty..
A good place to get a battery is Walmart, good prices, good warranty... My experience tells me it's one of these two problems...Good Luck..