When we say something very usual, there is nothing surprising in it.  On the other hand, when something is being explained as unusual  an element of curiosity is aroused and someone is given to understand what is being said/ or expressed bears the meaning or connotation of being against the practice.  The following examples will further clarify the essence of the usage.

• He is very happy.  He is satisfied with his achievements.( This is a statement that shows a general state of affairs.  People are not surprised why he is happy)

COMPARE

• He is being very happy.( i.e usually he is unhappy and one is surprised to see him happy)
• I don’t know why she is being very angry with me.(i.e usually she is not angry with me)
• She’s being very nice to her sister at the moment, I wonder why.( i.e it indicates that she is not usually very nice to her sister)
• Usually Davis is a very sensible person.  I wonder why he is being so insensible to accept the proposal)
• I wonder why the manager is being so late today.(Usually he goes to the office in time)