I have used the Revolution for years. I have 2 in the under 5 lb and 2 in the 5-10 lb. dosages. The smallest amount is for all dogs and cats under 5 lbs. regardless of age...most dogs get bigger than that so they only put 3 vials in there. The 5-10 lb. box includes 6 vials. I think they have a different box for cats over 5 lbs. so that would explain the AND on the smaller box.
my 2 little ones will always be in the under 5 lb. size so i have to buy 2x as much. I asked the vet if i could get more in a box but they said that they only give 3 in a box because most puppies and kittens get over the 5 lb. quickly and then the vials wouldn't be enough.
We love the Revolution and the fact that it includes the flea/tick along with the heartworm treatment. None of my girls have every had a problem with it.