parent trasacation of responce time is not equal to sub trasactions of same psrent trasaction in Loadrunner of Vugen tool excuting

one activity have many requests is coming in VuGen .So I started main transaction and put sub transactions to every request. after execution in VuGen total of response time of parent transaction is equal to total of sub transaction of same parent transaction But it's not equal to response time of total.

For Example :

lr_start_trasaction("Application Name_Module Name_Login_01");
Lr_start_sub_transaction("Application Name_Module Name_Login_01_sub_01");
lr_end_sub_tranaction("Application Name_Module Name_Login_01_sub_01""LR_Auto");
lr_end_trasaction("Application Name_Module Name_Login_01","LR_Auto");

above the way I created script and total Response time of parent transactions is not equal to total sum of sub transactions.

Thanks in advance..

Thanks SivaK