A lot can happen in half a year to an individual, plus memories start to become foggy. Six months is a practical time limit factoring in personality fluctuation and brain recall function, no need to read anything more into it. If someone hasn't been active in that period, he can always send a note to the lady he last saw to freshen her memory and give her a heads up that a ref request may be coming her way.
Circumstances change and not always favorably, so it isn't unreasonable to expect a provider to re-screen a Hobbyist who has disappeared off the scene for years and has no recent refs.