Higher working pressures in the cartridge. Also, certain powders can exhibit larger pressure spikes/swings with changes in the outside temperature. A couple of examples are Reloader 15 and 17. Both are excellent powders, but can show considerable changes in pressures and resulting velocities across a temperature gradient. This is especially important to take into consideration when developing a load during the cool "hunting" months, and then shooting it again during the hot summer months. Granted, those poweders will give greater velocity, but like everything, it comes with a price.
This is absolutely correct. I shoot the 6.5L using re15/123 scenars and usually have to make slight elevation corrections in the afternoon as things tend to speed up. I have no experience with the 243, but I can tell you I have never shot anything as accurate as the 6.5L to 900 yds.