Bill,
This is also why you must regulate with a known and repeatable load, a handload, not a factory. Even if the factory speed data was correct, you still won't be able to reproduce it with a handloaded shell. If you did manage it, that would be extremely lucky. Many of the new factory loads that come out use a bulk powder that we can't even buy.

As Daryl mentioned, you can get varying points of impact by varying the loads, even with nitro powder. That is how the NfB loads work. You keep playing around with different powders and amounts till you get a safe load that will duplicate the regulation load used in your gun at the factory. Usually this load will have about the same speed and the same barrel time as the original load, if you are using the same bullets as the factory did to regulate it.

Any change will usually change the POI of your bullets. If you go from cast to jacketed, if you start using a different primer, or in some cases, if you start using powder from a different lot, those can all change your regulation. It is also not true that there is only one regulation loading for each gun. Some may be more picky about what you load them with, but because of all the variables we have today, it is not unusual to be able to work up at least two or three hunting loads using different bullets, different powders, primers, or all of the above.

You will note that I said hunting loads, and what I will accept, and what you want, may be two different things. But be aware that none of these are target loads, doubles are not target guns. They are made to throw two bullets into a decent hunting sized pair, that's it! The very best you can do to regulate a double is to get them printing on the same exact elevation and the same distance apart, C. to C., as the bores are at the muzzles.
Bob