Comparison of 2 models with observed data - identical R2?


I am trying to compare field data with 2 models, but somehow both models give me identical R2. The models are derived from different sets of data so I am not sure how this is possible! Even to about 7 decimal points, for more than one independent variable, the R2 is the same. (I am using excel's "=RSQ(x:x,y:y)" formula). Is there some explanation for this? Or did I make a mistake somewhere?