Accent on ουκ εστιν

[Cheiromancer]

I've consulted Goodwin, Smyth and Chase & Phillips, and as far as I can tell they all agree that when a proclitic (like οὐκ) is before an enclitic (like ἐστίν) the proclitic has an accent and the proclitic has none. However a search with the Perseus database reveals that

οὔκ ἐστι occurs 5 times
οὔκ ἐστιν occurs 6 times

οὐκ ἔστι occurs 394 times
οὐκ ἔστιν occurs 770 times

And so the rule appears to be followed very infrequently, at least for this pair of words. Does anyone know what is going on here?

[Cheiromancer]

annis just posted a lovely grammar guide from the University of Dallas, written by one Karl Maurer. On page 7 he gives an unusually complete list of the circumstances estin receives its accent. The rule that covers this case is "if it follows οὐκ, μή, εἰ, ὡς, καί, ἀλλά (ἀλλ') or τοῦτο."

I wonder if there is a more intuitive way to grasp the rule?

P.S. The full text of Maurer's rule for estin is

SPECIAL RULES FOR ἐ σ τ ί : accent it ἔστι (A) if it is the first word; (B) when it means "it is possible" (ἔξεστι); (C) in the phrases ἔστιν οἵ, ἔστιν ὅτε etc.("there are those who" = "some people", "there are times when" = "sometimes"); (D) if it follows οὐκ, μή, εἰ, ὡς, καί, ἀλλά (ἀλλ') or τοῦτο.

edit: Of course, this doesn't explain what is going on with ouk eimi. The special rule for esti doesn't apply to it, so it should appear as οὔκ εἰμι. But it doesn't. It is instead οὐκ εἰμί. Maybe those same 7 words are exceptional for all the bisyllabic present indicative forms of eimi?