why do we say that meaning of a sentence is not the sum tota

The meaning of a sentence is not the sum total of the meanings of all its components because it cannot be worked out by adding up all the meanings of its constituent words. For example;
(A) The dog bit the man.
(B) The man bit the dog.
If the meaning of a sentence were the sum total of the meanings of all its components, then the above two sentences would have the same meaning. In fact they are different in meanings.
As we know, there are two aspects to sentence meaning: grammatical mean­ing and semantic meaning. The grammatical meanings of “the dog” and “the man” in (A) are different from the grammatical meanings of “the dog” and “the man” in (B). The meaning of a sentence is the product of both lexical and grammatical meaning. It is the product of the meaning of the constituent words and of the grammatical constructions that relate one word syntagmatically to another.