There are a few words that follow the same pattern, for example prescribe/prescription. It's just a spelling change between the verb and noun forms. My guess as to why would be because pronouncing "describtion" clearly is pretty difficult, so it just turned into "description" over time.

What?

Because our spelling is (fairly) phonemic (meaning it is written to represent the spoken form (unlike Chinese, for example)). Each character (for the most part) represents a different phoneme (/b/ is a phoneme, and /p/ is a phoneme). Therefore: Big and Pig are written different because they both have two different sounds (/bɪg/ and /pɪg/).

Please give an example