Why is it "description" but "describe" (b and p)?

Solution 1:

"description (n.) late 14c., from Old French description (12c.) and directly from Latin descriptionem (nominative descriptio) "representation, description, copy," noun of action from past participle stem of describere "write down, transcribe, copy, sketch," from de- "down" (see de-) + scribere "write" (see script (n.)).

and

"descriptive (adj.) 1751, from Late Latin descriptivus, from descript-, past participle stem of describere (see description). Related: Descriptively; descriptiveness.

and

"describe (v.) early 13c., descriven, from Old French descrivre, descrire (13c.), from Latin describere "to write down, copy; sketch, represent" (see description). Reconstructed with Latin spelling 16c. Related: Describable; described, describes, describing."

All from: http://www.etymonline.com/index.php?allowed_in_frame=0&search=description