Hi JDH ...
Dredging down on your experience with frost ... I have noted in vines that are subjected to cold spring-frost conditions, between shield splitting and bud-burst, that the number of both clusters and florets is reduced. It is varietally variable with Sauvignon, Shiraz and the claret varieties being more affected than Riesling, say.
Sauvignon being the predominant variety - I've noted sites that are warm over spring have a mean of 1.6 clusters a shoot and set 80 berries a cluster; plants in a cold site these have reduced to 1.2 and 68 respectively.
Now the sites are not greatly distanced and growing on dormant canes (by bud) shows no statistical differences - my assumption then is that the significantly colder spring causes (a) clusters to simply abort and (b) a depression in floret number. The former is of greater influence on tonnage of course - but cumulatively, a significant reduction in yields.