I never really paid much attention to this before, but yesterday I went to the range with 2 different brasses loaded exactley the same and 1 type brass shot 100 fps faster than the other. Same col,aol,buller powder and primer. Can someone explain this to me? This may be a dumb question and I just need some clarification.

Usually a result of less case capacity in the "faster" cases. Smaller "boiler room" same powder charge,bullet, primer, = more pressure and sooner. Possibly affected by harder brass, but this isn't usually notable.
To confirm this fill a few of your empty cases from each lot with water to the top of the neck(with primer still in place). Then weigh each one. The difference in weight between the case lots is the difference in case capacity fast lot vs. slow lot

muck.
This is one of the reasons you should reduce your powder charge and work back up when you change components.

Muck

You may get the same total weight for each case with your method. You need to take an empty case with primer still in it, weigh it, then fill with H2O and weigh again. The difference is the capacity. I'd do it with at least 2 or 3 cases and average the capacity.

Of course, all that tells you is that one case has more or less capacity than another. It does not tell you anything about how much powder it will hold, or about pressures, etc.

Ray

How many shots over the chrony of each did you record?

i agree with all the above comments. I usually use a real fine powder instead of water to compare to cases for capacity. AND as Vapo says...how many times did this happen...that's a pretty big jump in velocity for a "strictly case capacity" situation.

20 of each brass, 20 LC and 20 AP the AP recorded 100 fps faster consistently in 3 shot groups, but thr LC grouped better.