Stijn, thank you for raising the point. I have just recently joined Clcik dimensions and had exactly the same issue calculating Click rates.
Matthew, correct me if I am wrong on the following.
The Statistic Graph on email send represents the following:
Click Rate, which is calculated as follows: Unique People Clicked/Sent Emails
However, Unique Clicks represents a value which is different to what you are using in the Click Rate, so not Unique People but unique clicks per link. Why do you use different values on the same report?