BSCI - Multicast - 224.0.1.40, where did that come from??

When you are configuring PIM in Sparse-Dense mode every router automatically subscribes to the Rendezvous Point Discovery group 224.0.1.40 .

This why when you do #sh run you may have entries for:
!
ip igmp join-group  224.10.50.1
ip igmp join-group  224.10.70.1
!

But when you do  - #sh ip igmp groups
You acually get listed:
Group Address
224.10.50.1
224.10.70.1
224.0.1.40

Confusion cleared!