The local grocery store sells both jelly and peanut butter at very reasonable prices, but each peanut butter jar is roughly 2/3 the size of a jelly jar. If, by making peanut butter and jelly sandwiches, you consume peanut butter and jelly at a roughly equal rate, you'll have 1/3 of a jar of jelly left over when the peanut butter runs out.
This has happened to me several times. One solution to this problem is buying 2 jelly jars and 3 peanut butter jars at a time so that everything evens out in the end. An interesting question, then, is whether the jar size discrepancy is a coincidence or a purposeful move to increase bulk purchases from the peanut butter and jelly sandwich demographic. I'm not sure if PBJ is the same cultural phenomenon in the UK as it is in the US; maybe the extra 1/3 jelly is meant to be spread on scones or something like that.
Funnily enough, the jar of peanut butter costs more than twice as much as the jar of jelly. Are peanuts that much more expensive than strawberries, or is it something about the processing or other ingredients? The speculation never ends.