Some NPHC members have the idea about NPHC membership this way: "If you're in, then you know. If you're not in, then you don't need to know."
NPHC orgs are involved in philanthropy - we just call it by a different name, community service. That is probably why you are getting this look
from your NPHC family members. Ask them about their community service projects. Or, better yet, look up the community service projects of the D9 orgs on their international websites.
That's another philosophy behind D9 membership - if you want to be a member you will do your own research to gain knowledge, and not ask anyone questions you should be able to figure out on your own.
Back in the day, early 1960's, when my mom was an undergrad, the DST chapter on her campus (Texas Southern Univ.) would invite a young lady to membership by tapping her on the shoulder. Up until the early 1990's, a person could not become a member of a D9 org, or 'pledge', until their sophomore year in school. So that meant the D9 chapter had a year to 'check you out', so to speak. If they liked what they saw, you were extended an invitation to membership, if they did not like what they saw, then you never got the invite.
For the most part, the way D9 orgs handle membership today is there are about 2 -3 open invitation information sessions. There you will leave your contact information. If the chapter is truly interested in you, then you will be invited to a closed (invitation only) information session. If the interest is still mutual, then you will be given the specific information for membership. Usually, at this point you are given information about dues and MIP (membership intake process).
I think the big difference you see between NPC and NPHC is how we handle our recruitment process.