Actually Howrah is the big Terminus and all south bound train even North bound trains begins from howrah. And in case of sealdah section there are two section South and Main. South is mainly for suburban train moving towards Diamond Harbour, Canning, Budge Budge etc. And Terminus of South and Main of Sealdah is perpendicular. And sealdah and kolkata is well connected with Chord and main line of Howrah which goes towards Bardhaman and it is obvious that Dhn bound train gets easily connected.
Now for South bound train from Sealdah. Only Sealdah Puri Duronto Runs from Sealdah. And it uses Bhattanagar ACC route. So only countable number of trains run through Bhattanagar ACC.
Now...
more... origination of south bound from SRC, reason is that SRC lies between HWH-KGP route. And every train move towards KGP side has to cross SRC. So to reduce congestion they begin from SRC.