Actual Vs Est

It is derived from the estimate of the number of rows that will join (the choice of physical join, merge, hash or nested loops does not affect the cardinality of the result, just the way it is displayed). The estimate for the join output is not brilliant, but that's most likely just a reflection of the limited number of histogram steps available. SQL Server generally estimates a join by aligning the steps of the histograms from the two inputs. The histograms are necessarily an approximation, and so is the result.