Article "a" or "the" ?

Always not the case. Definite article "the" or indefinite "a" can be used with truth or lie.

Carol that’s really a good q and Unique observation

Try this:

http://www.google.ca/#hl=en&gs_nf=1&pq=article%20why%20a%20lie%20but%20the%20truth&cp=9&gs_id=dw&xhr=t&q=definite+article+why+a+lie+but+the+truth&pf=p&sclient=psy-ab&oq=definite+article+why+a+lie+but+the+truth&gs_l=&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&fp=c202ea380386335d&biw=1588&bih=857

And Carol please let me know if you have any luck. For the life of me I’m never able to find the phrase in a document that’s quoted in the excerpt

Could t be because “a truth” conveys a subconscious implication that some of the cognate assertions might might be lies

I’d considered euphonics inasmuch as “It’s the lie” is more of tongue twister than “It’s a lie;” but then “It’s a truth” is easier than "It’s the truth”

Or maybe some combination of factors—yes, subtle