ABS vent joined to inside of other ABS vent pipe

I've never seen this which speaks to my lack of experience not the rarity of this. I have been trying to reroute a vent pipe for a vanity sink (see other thread). When I checked out the venting I noticed that one connection, near the wall where the vent goes to the sink, the pipe was joined by having one pipe (the wall pipe) inside the other pipe. It looks like the one pipe was flared open and the other pipe was inserted. A couple of questions pop up: First, why would they join pipe like that? Second is there an adaptor fitting available to make a connection?.

Thanks