Or is it?

Certainly we've seen many a fine Colheita come from lackluster years in which vintage Ports were either not declared at all, or scant few were produced. Significant Colheita years like 1986, 1974, 1964, 1957, 1952, 1944, 1940, 1938 and 1937 all come to mind, to name a few good examples of mediocre vintages that produced some excellent Colheitas. Given that there is about half as much Colheita produced as Vintage Port ... we are talking about a rather scarce Port category to begin with. Heck, you are hard pressed to even find a decent selection of Colheitas anywhere in the UK, while VPs abound there.
But then there are certain vintages ... a few of which were great for Vintage Port too, that made exemplary Colheitas. 1963, 1966 and 1967 all come to mind as well as 1987 and 1995 more recently. But when I think of a year like 1937 in which almost every major producer of Colheita just happened to launch their fine vintage-dated-Tawny Ports (a.k.a. Colheita) I scratch my head and wonder what caused this phenomenon.
So is there any real correlation between Colheita production and the vintage it comes from?