Sometimes this crop adjstment is done later in the year if there is the thought that the gapes on the vine are not going to ripen adequately before the end of season. The second cluster may not ripen to the same level as the first cluster and if harvested together, the higher quality first cluster is averaged lower with the lower quality second cluster, potentially lowering the quality of the wine.
When cluster pruning, the second or third cluster is usually removed which concentrates ripening into the remaining cluster.
We have tested about 30 varieties of grapes at our vineyard and it seems as though some varieties ripen the first and second clusters similarly while other varieties show significant differences in ripening between clusters.
See St. Croix below, with visually obvious differences in ripeness between first and second cluster.
We can usually tell the years that inadequate ripening may be an issue by the timing of veraison. Late veraison means that we may not have enough time and heat left in the season to ripen the grapes to our liking. So thats when we would consider cluster pruning of the second cluster.
In the past ten years this has happened only once..but this is also because after 10 years we know what the crop load should be for the various varieties we have as it relates to our vineyard climate and conditions. And we also know when the usual timing of veraison is for the various varities.