Comment 2 for bug 311175

It's one possible reason; simple concurrency would be another. In theory, concurrent SERIALIZABLE transactions would run in a near-perfect illusion of being the only transaction currently executing. In practice, that's expensive to achieve so postgres approximates it. (It's recently been discovered that this approximation doesn't quite meet the standard).

That means that there may well be some cracks in this case that occasional inconsistencies can sneak through. As long as those are few and small, and the cron job catches them anyway, trying to fix that is probably not worth the effort.