How common are 51R batteries ? That will be a factor too. If there's another battery that's just a bit smaller, they may have chosen to substitute the smaller one. It will fit (may not be optimal on CCA, etc, but that's their decision) and they stock (1) battery vs (2).
Originally Posted by user52165
"The other thing is that Costco saves money by limiting distribution. In my are they don't have any Group 51, although they have 51R" It's not just distribution with Costco, size of store matters. There are 10 Costcos in my area and none carry exactly the same of anything from cereal to batteries etc. Try a bigger Costco, you might find it.