Some PTR sites only have a small number of advertisers so have less to send out, of course

If people aren't willing to advertise somewhere, then you'll have less emails to read.
Being active basically means clicking your email links / PTC links regularly. When a site says that it sends "an average of X emails a day to active members" then this can depend on a few things eg:
1. Where you live
2. What interests you have selected
3. What advertisers want
Some advertisers choose that emails only go to certain demographic groups (ie Tier 1 countries only to maximise PTP earnings), and others only want adverts to go to people who have selected "I like eating bananas" (eg) as an interest.
So the number of emails can vary.
Of course the PO may not be totally honest - this sometimes happens, and the information may not be true, or it may have been true a while ago, but its just not up to date.
TYhe only thing you can do to find out more is to ask each specific PO (PO = Programme Owner, by the way) for each programme you are a member of, and that will give you the defenitive answer for each particular site.
Hope this helps