It is Russian orthoepic norms. http://ru.wikipedia.org/wiki/%D0%A7 http://fonetica.philol.msu.ru/orfoepija/tabl/chn.htm

Some words are in "A" and pishim "O" ,"T" and "D", "З" and "C"

Hello) I am not a teacher of Russian language, but I can say that there are a lot of accents , slang words in the system of russian language, there are also many accents , it depends on a part of our country! For example, as it' s known that in the North and in the Volga region people pronounce the words with ' o' letter, in the South they pronounce them with 'h' ( где -hde), in the West and East we can hear very firm 'ch' ( скучно -skuchno) but i pronounce this word like 'skushno' , think that it's correct way , it's hard to bring together all the peculiarities of pronunciation of russian words... Anyway, i' d like to add that the ' right' form of pronunciation is considered to be Moscow form )) or it comes and sets from Moscow ...but you should know that Moscow is a city where you can meet a lot of people from different cities of Russia, and even we joke there are no real aboriginals in Moscow, only people from periphery)

Simply, the reason is little "laziness" in pronunciation of sounds. Consonants are "deaf" and "ringing". So "deaf" are a little more easy to pronounce. So we say them against "ringing" in the middle of most words and sometime at the beginning.

It's Russian pronunciation :)