Why is it 'three score years and ten' almost half the time and not always 'three score and ten years'?

It is an idiomatic expression as the Phrase Finder suggests:

'Three score and ten' is the nominal span of a human life. In the days that this expression was coined that span was considered to be seventy years.

Threescore used to be used for sixty, in the way that we still use a dozen for twelve, and (occasionally) score for twenty. The use of threescore as a name for sixty has long since died out but is still remembered in this phrase. Threescore goes back to at least 1388, as in this from John Wyclif's Bible, Leviticus 12, at that date: "Thre scoor and sixe daies."

There are numerous uses of 'threescore' in the Bible. Most of them refer to its simple meaning as the number sixty, for example: "...threescore and ten bullocks, an hundred rams, and two hundred lambs: all these were for a burnt offering to the Lord."

See usage frequency of “three score years and ten” vs “three score and ten” in Google Books

"Threescore years and ten" is a quotation from Psalm 90:10 in the KJV. That explains how come that exact wording, with the noun "years" before the "and" rather than at the end, is so common. Also how come it is used at all, long after it was anything like customary to express numbers 40 and up in terms of scores.