How did "credibile est, quia ineptum est" become "credo quia absurdum"?

It looks like a paraphrase made by the Zen Buddhist D. T. Suzuki's book, "Introduction to Zen Buddhism". Although, D. T. did use a different phrase beforehand, and it's possible that the paraphrase came before this writing (which was in 1914). The line quoted by Suzuki comes one line after the original line in question in Tertullian's work, so they are quite closely related. As Suzuki suggests, this phrasing may make more logical sense. It also is shorter, and thus lends itself to brevity.

http://en.wikipedia.org/wiki/Credo_quia_absurdum

About the phrasing

I am not a Latin Scholar, but I had three courses of Latin in High School in addition to extra-curricular Latin interests and exploration. I can say that, for the original quote, the repetitive usage of "est" (He/She/It is) is likely annoying and/or feels rather choppy to those who speak and read Latin fluently. Often times, the verbs of the form "to be" in Latin are implied, either by other verbs or simply by the mutual understanding that they are there (which can make it more difficult for those of us who don't read Latin on a regular basis to understand). I am unsure, but this could be (in written Latin) due to the fact that ancient man needed to use his paper-like resources carefully, since they did not have paper sold for less than a cent per page back then. It was a much scarcer resource.

I recommend you to read this article, because it specifically addresses your question: “I Believe Because it is Absurd”: The Enlightenment Invention of Tertullian’s Credo", Church History, 86, 2017), pp. 339–364.

In brief: it was a misquote originated in the Anglophone context of the seventeenth century where it became "credo quia impossibile est", and it was later popularized by Voltaire for the first time in Le Dîner du comte de Boulainvilliers (1767). But here, he attributes the quote to Augustine, and he also introduces the element of absurdity ("St. Augustin parle par économie quand il dit, Je crois parce que cela est absurde. Je crois parce que cela est impossible"). It's very likely that he misattributed the quote on purpose.

Quoting the abstract:

Tertullian is widely regarded as having originated the expression Credo quia absurdum (est) (I believe because it is absurd) and the phrase often appears in contemporary polemics about the rationality of religious belief. Patristic scholars have long pointed out that Tertullian never said this or meant anything like it. However, little scholarly attention has been paid to the circumstances in which this specific phrase came into existence and why, in spite of its dubious provenance, it continues to be regarded by many as a legitimate characterization of religious faith. This paper shows how Tertullian's original expression—“It is certain, because impossible”—was first misrepresented and modified in the early modern period. In seventeenth century England a “credo” version—I believe because it is impossible—became the common form of Tertullian's maxim. A further modification, building on the first, was effected by the Enlightenment philosophe Voltaire, who added the “absurdity condition” and gave us the modern version of the paradox: I believe because it is absurd. These modifications played a significant role in Enlightenment representations of religion as irrational, and signal the beginning of a new understanding of faith as an epistemic vice. This doubtful maxim continues to play a role in debates about the cognitive status of religious faith, and its failure to succumb to the historical evidence against it is owing to its ongoing rhetorical usefulness in such debates.