Why is the spelling of "pronounce" and "pronunciation" different?

Why is the spelling of pronounce and pronunciation different? If one originally did not know the spelling of pronunciation, one would when hearing it verbally deduce its spelling to be pronounciation, which is the incorrect spelling.

Why is the difference in spelling so? Also, are the two words pronounced differently? (I think I am using the American pronunciation for pronunciation (pro-noun-ciation), but according to howjsay.com, it's pronounced as pro-nun-ciation. Is this just for British English or universally?)

Solution 1:

pro-NOUN-ciation is universally wrong. Even the highly permissive Merriam-Webster dictionary marks it with an obelus (÷). Here is what they say about pronunciations marked with an obelus:

The obelus, or division sign, is placed before a pronunciation variant that occurs in educated speech but that is considered by some to be questionable or unacceptable. This symbol is used sparingly and primarily for variants that have been objected to over a period of time in print by commentators on usage, in schools by teachers, or in correspondence that has come to the Merriam-Webster editorial department. In most cases the objection is based on orthographic or etymological arguments. (source)

As for why the word pronounce has an O between the two N’s and pronunciation does not, it is unclear, but both words derive from French, pronunciation from pronunciation and pronounce from pronuncier. There is probably some variation in the way the different word stress affected how the words were spelled after being borrowed into English.

Solution 2:

Trisyllabic laxing is the reason for the pronunciation difference, which led to the spelling difference.