For example mixing countries into your targeting may mean you pay too much for some country traffic and not enough for others. For example if you are selling software solutions globally, does it make sense to bid $2 worldwide? $2 will see you at position 1 all day for many countries and 9+ for others.
What you can do to fix it:
A single bid will often lead to skewed country data and if you don’t believe me, check your Dimensions tab in your previous or current multi country campaign. Grouping countries into ‘segments’ is a way to avoid this and will allow you vary your bid across multiple countries to ensure you get traffic and data from all the applicable regions to determine what areas are profitable, and which ones are not.