The more powder, the more energy it produces,the more energy the higher velocity.
When you see MAX LOAD it means exactly that. Maxloads should only be approached slowly with the utmost of care. Some firearms seems to handle "max loads " better than others with different bullet weights.

That's why I can't stress enough....approach max loads with extreme caution.

I'm not familiar with loading a Weatherby but, when reloading specs are given they are also using a specific case, primer and barrel. These kinds of variations are not unusual. Go by what that particular manual says.
If you cannot gather the exact components and barrel. Start at the minimum load and work up to pressure signs. Once you become familiar with the processes. You will be able to understand which and what you will be able to substitute.
Do not assume that a variation or substitution of a component is compatible to the rifle you are using. If you substitute a component. Start at the minimum and work up to a powder charge that suits your shooting requirement.
Stick with it. It's great fun.

Hammer it just dont look right when 72.2 gr of powder will go faster than
76.7 gr with the same powder and same bullet,
I think fuzzy may have answered it by the possibilty of 2 different kinds of cases
in each manual Thanks

The reloading manuals loads are accurate only in the chamber they were developed in and the condition's that existed when they were developed. In other words, these are only guide lines. You'll notice that every manual will tell you to start at least 10% under the max laod and work up watching for pressure sign's.

Each chamber is slightly different as the same reamer is used to cut a number of chambers until the reamer wears down to min SAMMI spec.'s. Then it's discarded and a new reamer used. The first is a max chamber and the last a min chamber. Both will give pressure differently. That's one reason to never use a hand load that was developed in someone elses rifle. There's also very small differences in the barrels that may raise pressure.

When the manual writter does the manual all these thing's need to be accounted for so you'll find variations in all of them. Just remember that those manuals are mearly a guide. Start low and work up watching for pressure signs. All good manuals will tell you what to watch for and show examples in photo's.

ok that makes sense each case will expand differently to the chamber they are in producing different pressure and different velocity is that what's happening! thanks don

I just noticed that the reload data I have for the 300 Wby is for 26 " barrels.
Does Weatherby even make a 26 inch barrel ??

I suspect that the barrel length for each manual is different.(ie. 24" vs. 26" barrel) Another unfortunate possibility is that Hornady is elevating the speed of their bullets for commercial purposes. I don't worry too much about stated velocities because for the most part they are inaccurate. The only way to compare your loads is to chrono. them.

I belive The big W makes all there barrels 26 inches as standard.

It looks like 24" and 26" are both stardard sizes on their web site and seems to depend more on caliber.

That's pretty close to right. Remember that Temperature also play's a role. Loads worked up in freezing weather may well be to hot (pressure wise) in hot weather.