Looking at "Actions > Sites" the percentage values are not summed up correctly.
If for example a group has 4 child entries with (100%, 0%, 0%, 0%) then the complete group gets 100% instead of 25%.
Tested with version 0.9.9.
We don't have a menu/submenu item for "Actions | Sites". I assume you're referring to bounce rate (and not exit rate).
I looked at the calcs and they look ok (bounce_count / nb_visits).
I should point out that the aggregate bounce rate for the category is not a simple average (eg 100 + 0 + 0 + 0) / 4. Piwik keeps running totals so it would be equivalent to a weighted average of the child percentages and visit counts.