culpable homicide is not considered murder

Ma’am,

I understand your query and would like to answer in the simplest way assuming you’re a layman.

There is a very thin line between murder and culpable homicide. All murders are culpable homicides but not all culpable homicides are murders.

In the Neeraj Grover case, you have to consider the degree of the intention of offenders. If the person is killed in cold blood or with planning then it is murder because the intention to kill is to a high degree and not out of sudden rage or provocation. On other hand, if the victim is killed without pre-planning, in a sudden fight, or in sudden anger because of somebody's provocation or instigation, then such a death is called culpable homicide. 

Hence, here the act occurred in a sudden fight/anger, therefore it is culpable homicide. I hope I've answered your question. Thank you.