The 40 would be a breeder - it is 36Lx18Wx16H. Really nice dimensions, not tall and thin and difficult to work with. There is a 40g long - that is a shorter 55g, but not one most stores carry and not considered a "standard size" for the sale.
I believe they usually do 10g, 20g L & H, 29g, and 40g Breeders. I think some stores may do 5.5g as well. I believe they usually word it "up to 50g, standard sizes" to avoid having the 55g on sale as well. Some stores also only stock (or only put out) a few of the larger sizes, so they aren't selling too many 40B tanks at $40 a pop.
FWIW, the 40B is usually at least $115 new. Even the new 20L is like $44 I believe, and a 29g is about $60 IIRC.
I'm sure they take a loss on the 40g at least, prob the 20-29g as well. Not the 10g. Most people will at least get the hood and light with it, usually also filter, heater, stand, gravel, etc.
At their last $/G sale, I got a 40B, a 20L for a sump, and a 10g QT tank. And a bag of salt, no other accessories. So I'm sure they lost a crap-ton with me. And they also helped carry it out to the car