Colo(u)r me a confused Yank.

I did get an AA Smartfuel card but found out from a random Caltex cashier that I'd been using it wrong. She told me to fill up in $40 increments and even to do two separate, consecutive $40 lots before going inside to pay to maximize the rewards.

I did that at a BP today, and the cashier acted surprised when I said I wanted to spend rather than accumulate rewards. He then said, "All right, I'll combine them into one big transaction, then," and I proceeded to get just a normal 4 cents off per litre.

Where am I going wrong here? What should I do to maximize benefits? When should I spend vs. accumulate discounts, and should I divide up transactions like that?

Hoping a knowledgeable Kiwi can help this befuddled American out...our fuel rewards programs in the U.S. are much simpler, though also apparently much less of a good value--like 3 cents per gallon, max.