I get H is teenager, but i also don't think he would confuse five minutes with one hour.
Supporting the longer shop time is the fact the boys had to bring the groceries in and then CB and/or CH had to sort through and determine what groceries belonged to whom. If all CB got was milk and cheese, then she could tell the boys to take those items to her refrigerator and everything else to CH's camper. (I know CB said there was one more thing that since then she forgot, well, at the time she would have known so this covers whatever that was, too.)
Time seems to have been maximized on some stops, but was minimized at the grocery store. Idk... Just seems odd.