tark, yes, over the years, I have noticed what you're talking about, with both the Super and 10MM. I suspect there is more than one reason for what we're seeing. I have old notes on chronographing the Super back at least as far as the mid '80s. More recently, I've chronographed Winchester, Remington and Federal 130 grain ball at between approx. 1140 and 1240 FPS in 5" barrels. I also chronographed the Winchester 125 Silvertip in three different pistols with 5" barrels. Only averaged ~1160-1175 FPS. It's advertised at 1240 FPS, IIRC. Among "Boutique" manufacturers, I've only tried Cor-Bon's 125 JHP in the Super. I tried the CorBon in a Kimber with 5" barrel. It averaged about 1315 FPS. I had an indentical Kimber in 9MM, and tested Cor-bon 125 JHP 9MM the same day. The 9MM averaged about 15FPS faster then the 38 Super. Not the results I expected...
I've reloaded the Super for quite a few years. Some factory 38 Super brass I sectioned was VERY thin in the case head area. I can underatnd why they would want to load the Super down given the case construction. But, never could understand why they produced brass so thin in this vital area. That same manufacturer produced the thinest 9MM cases I ever sectioned at both case mouth and base. Even so, that manufacturer's 9MM cases were more robust in the area of the case head than in the Super. Things are better with the Super now days IMHO, since we have things liked ramped barrels and .38 SuperComp brass,etc. Anyway, the Super is capable of being safely loaded to velocities and energies quite a bit more impressive than displayed by most major manufacturer's offerings...ymmv
Ps, I agree with tipoc on the 38Super.net site. I have visited there several times. It contains the most comprehensive and detailed information on the 38 Super I have ever seen...