OT: Andrew will have a pretty good idea of when his flock will start lambing - you just add 145 days to the date of conception. You can't assume that the tup(s) will 'do the business' on the day when they're first let into the ewes' field, so the tups are fitted with raddle markers (a block of greasy paint on a body harness, so that the paint marker is under their abdomen) so that when a ewe is served a coloured mark is left on her rump. These are changed periodically, so you can tell, within a week or so, when a ewe ought to be lambing; and in a big flock you may stage them in groups of a hundred or two at a time, from out-fields to near-fields to lambing pens on the yard. Raddle marks are also useful as you can see which ewes aren't likely to be in lamb and can move them to leaner keep and keep all the good keep for those lambing productively. (Its a lot easier to use, and particularly change, the raddle nowadays - it used to be just a mixture of tractor grease and powder paint).
There is though Jeff, what I suspect you're seeing, variation by flocks as a result of climate or a particular farmer's practice or risk profile - early lambing can be more vulnerable to bad weather, and due to colder weather there's less good grass on the fields so more hay & other fodder required, so costs higher & returns possibly lower for early lambs - these are all weighed up when deciding when the tups will be put in with the ewes.
Dave