Only when they are doing a spread "exclusive" inside the mag itself will they have to do more than one pic.
Street shots by PAPs are normally several in succession and aren't that staged Betty. Just look at the Daily Fail for examples when stars get papped. They have several pics of them walking along, not just one pretty happy family shot.
Maybe one pic was was deemed "good enough" for an exclusive in their mag?
or maybe there was more than one and they used one