Sometimes its because the number doesn't match or make sense with the sentences/topic causing the sentence to not flow smoothly. & you also say the numbers different when counting things [they count by counter]. Another thing is the number 4 & 9 also means death & suffering in japanese which is unlucky [or any other asian culture].

this question is difficult for me When I reading 1 2 3 4 5 6 7 8 9 10 I say ici ni san si go roku shichi hachi ku jyuu but 10 9 8 7 6 5 4 3 2 1 jyuu kyuu hachi nana roku go yon san ni ichi Because I count easily when the number is counted. this is no answer of your question.... and Do you know most of Japanese Kanji has reading of two kinds or more? ON yomi and KUN yomi sorry my English is poor my explain is bad.... I'm sorry

4（yo),7(nana),9(kokono(tu), are old fashioned..And 4(shi),7(shichi),9(kyu)are general. It depends on the case which one you prnounce. This page is good for you. But these are Japanese.. http://www.rd.mmtr.or.jp/~bunryu/nihon2.shtml

: ) Take it easy . Even native speaker doesn't know the reason why they are pronounced differently. But , through general life, we can know how to count differently depending on the measurement unit. So , you should study each change to each measurement unit to be precise. the number of Book , Tofu , Time , Length , ..etc many !!! http://www.freejapaneselessons.com/lesson05.cfm http://answers.yahoo.com/question/index?qid=20080520072344AAylWPO

because one of them came from ancient chinese.