Hierarchical regression query

I am running a hierarchical multiple regression with a pair of independent variables. I have run this analysis with two different variables added in second whilst maintaining the primary independent variable, and in doing this the two analyses result in different R2 values being returned for the initial model with a single predictor. The R2 in one of the cases does match the value given for a simple linear regression of the dependent and first independent variable. Can you please advise me why the R2 values at the first level of the hierarchical analyses do not both match the simple linear regression?