The suggestion is maybe part of the sky is easier to survey, so it was completed cheaply, even though that part wasn't the most likely part. I don't know which

The suggestion is maybe part of the sky is easier to survey, so it was completed cheaply, even though that part wasn't the most likely part. I don't know which way it is, but it's an interesting question, and that's what OP is getting at for understanding the relevant probabilities instead of the relevant area.