This whole speed thing is a very interesting phenomenon. When I was searching the web for rides in michigan, I found the local club's forum and saw exactly the same issue being discussed. Clearly it's not just a PV thing. I think part of the problem is that there is a whole spectrum of speed out there, and people's abilities don't just fall into discrete 2MPH bins. In any of the groups, you are going to have a couple who can and want to push the pace up to the high end of the band and a couple who really struggle to hang on even at the stated average. Now add in the fact that we all have good and bad days, and well, even if we think we are in the right group sometimes we get dropped, which just makes a bad day even worse. It's those days though that make you a stronger rider, and they're just part of the process of improving.
Then there are the sandbaggers. By that, I don't mean lazy slobs like me who will ride outside of the H&N group because I'm dreading Sunday's track workout, but the few guys who feel the need to show the 21's, 19's, 17's, or even 15's just how strong they are. There are a few techniques a reasonably strong ride leader can use to fix this problem without ever having to say a word. I can almost guarantee that when these guys shoot off the front, or start ramping the pace up, not only do they not want to do a whole bunch of work by themselves, but they probably can't hold that pace by themselves either. In most cases, all the ride leader has to do is to make an authoritative move to get up close to the front, preferably just behind the offenders, and just sit there at the stated pace and let a gap open. 9 times out of 10, the sandbaggers will get the message and slow down. If they keep going off the front, or a large group splits into two, the problem has solved itself. Having the ride leader work him or herself between the "fast guys" (think separating children) can work too. And, if worse comes to worst, I've heard that sometimes a ride leader might pick a hotspot or hill to show the sandbaggers that they're not quite as fast as they think they are (I've never personally seen this happen though :) )
There is another important question to ask though, and that is why are people getting dropped from these groups? In most cases I've seen lately, it isn't because the group is simply moving too fast. Instead, riding with the 21's and 19's what I've seen is a conspicuous lack of smoothness. Most of it happens as people transition on and off the front causing the pace to vary up or down by 2-3 MPH. What follows the sudden pace change is a whole cascade of fumbling through the rest of the group, ranging from annoying yo-yo amplification to dangerous half wheeling to people darting out into traffic and the back of the group compressing and spreading out across the road. The poor riders working to hang on the back of the pack suffer the most from this, having to brake and accelerate over and over again until guess what? That's right, one of those accelerations opens up a gap that they just can't close.
So, what I'm really trying to say is that there is a a sort of magic to a really smooth paceline that makes the difference between a 19 rider being able to go home floating high after cruising along for 40 miles at an effortless 21-22 MPH pace, and somebody having a really bad day getting dropped at 20 MPH. I haven't felt much of that magic in the 19's or the 21's lately.
Earlier in the spring I tried a couple of experiments though. On a couple of rides that were mobbing the road, surging up and down, etc. I waited untile we were on a low traffic, relatively flat road and then got people going on a rotating double paceline. It was a little scarey at first, but it really got people to concentrate on how they were riding. With just a few minutes of coaching, things really smoothed out, and stayed that way for the rest of the ride.
Of course we have to be mindful of traffic conditions, but I think if all ride groups spent part of the Saturday ride doing some drills to actively improve smoothness, we would go a long way toward increasing safety and reducing dissatisfaction about pace variations.