I run a hierarchical regression in SPSS with two IVs (2 blocks) and one DV. I first enter my first IV and to see its unique contribution to my DV. then I enter my second IV to see whether there are any significant additive effects from the second predictor (my theoretical aim is to test whether additive effects-block 2 contribute significantly more variance to the outcome, compared to effects from one predictor only). In block 1 however, my IV has a different r2 value compared to when I run a separate simple linear regression with this IV predicting my DV of interest.. To my knowledge block one is just a simple linear regression between IV and DV and as such shouldn't it have the same r2 value with the separate simple linear model?