Not this or that, but this or this or this

Every time the gate is open is one of those phrases that comes from the old days when people built fences  to keep livestock from roaming into their neighbor's fields.  The gates let people and horses and wagons through but were to be shut after coming through them in order to keep the livestock in.  If the gate was left open, the animals got out, had to be rounded up and put back in their rightful place.  This happened frequently.

Once in a blue moon is a phrase that comes from a couple of hundred years ago when sailing the high seas was a major mode of transportation and exploration.  The moon had a blue cast to it every so often when it was full and there was a great amount of humidity in the atmosphere.  The phrase meant something happens infrequently.

When pigs fly is a phrase that comes from about the same time period.  It was coined when someone was certain that an event wouldn't happen.  Pigs aren't designed to fly anatomically, so it wouldn't just be rare that something would happen when pigs fly, it would be impossible.

Language is colorful and can be used for all kinds of reasons including taking a phrase and changing it from its literal meaning to something figurative.  These figures become well known and become embedded in language as idioms.  Idioms can also take on syntactic ideas.  In this case, all the idioms deal with time and are, thus, adverbial in nature.  And anytime a person deals with adverbs, (s)he can have gradations and comparisons - gradations in this case, from frequent to never and in between.

Life is not a dichotomy.  It is not black and white.  It is not always just right and wrong.  People don't have to decide something is either this or that.  Life has beauty to it and richness and spectrums and continuums and depth and height and flavors of sweetness and a thousand reasons for change.  Even our practice of language bears this out.  It is our pleasure to see that multivariate nature of life and not our narrowness to reduce life to right and wrong, black and white, this or that.