Why do we say "There is many a slip between cup and lip"?

Meaning:

Nothing is certain until it is completed.

Background:

The earliest surviving record of this expression in English is in R. Taverner's 1539 Proverbs of Erasmus; but variants date back to ancient times, such as in the writing of author and grammarian Aulus Gellius, who in the 2nd century AD recorded "much there is between cup and the tip of the lip."

One possible origin is found in Greek mythology. Before Ancaeus (a son of Poseidon and King of Samos) set out with Jason to find the Golden Fleece, a seer foretold that he would never taste wine from his newly planted vineyard on the island (which is still famed today for its wine). On his return, Ancaeus took a filled cup of wine and reproached the seer for the false prophecy, to which the seer responded with the proverb. At that moment, an alarm went out that a wild boar was in the vineyard. Ancaeus rushed out, leaving his drink untouched, and was summarily killed by the boar.

A different Greek myth also illustrates this proverb and is recorded by Homer in the Odyssey. When Odysseus finally returns from his extended journey, he finds a number of suitors vying for the hand of his wife Penelope. He quickly dispatches them, starting with the vile Antinous who "was on the point of raising to his lips a fair goblet, a two-eared cup of gold, and was even now handling it, that he might drink of the wine, and death was not in his thoughts." (from A.T. Murray's English translation of the Odyssey).

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