Currently you may be getting the following:
USA 1000 clicks @ Avg CPC $1.00 Avg Pos 13.5
UK 100 clicks @ Avg CPC $0.30 Avg Pos 2.8
Europe 100 clicks @ Avg CPC $0.30 Avg 2.5
In this example the vast majority of the clicks come from the USA so if all three countries are in the same campaign then the Avg CPC for this campaign will be close to $1.00 and Avg Pos will be close to 13.5.
What happens is that if you want to increase your traffic you will probably increase your Max CPC. This will increase your Avg CPC and Avg Pos for the USA, but do you really want to increase the Avg Pos of the UK and Europe campaigns?
By grouping your countries into the same campaign you can be pretty sure to over pay for a lot of your clicks.
Mods, please close this thread.