Aaron Bertrand walks through display differences between SQL Sentry Plan Explorer and SSMS:
Now, in the process of developing Plan Explorer, we have discovered several cases where ShowPlan doesn’t quite get its math correct. The most obvious example is percentages adding up to over 100%; we get this right in cases where SSMS is ridiculously off (I see this less often today than I used to, but it still happens).
Interpreting execution plans is not a trivial exercise, and this is an interesting look at how SQL Sentry developers (and supporters within the broader community) have worked on it through the years.