I want to know the reason on which concept its sounds in different but letters are same.
Roman script ABC is not unique to English. This is used for all European languages. Many words from Latin, French, German etc. are used in English. Also pronunciation of many words changed with time but spelling remain unchanged. Also the words borrowed from other languages could carry the spelling in source language but pronunciation changed. English developed in centuries. There is also variation between old, mid and modern English.
English is not a natural Science like Physics or chemistry with fixed law of nature. Language is more a matter of usage than logic. Hence, we have no way other than to accept spellings and pronunciation as used by English speaking people.
Let me also refer to my article on History of English. The link for the article is:
http://www.boddunan.com/education/61-English%20Language/15309-history-of-english.html