Because they are designed for use in a stock housing for the application for which they are sold.  The "calibration" is really only for a larger housing, because a stock sensor in a larger housing is not going to read accurately because of the differing flow dynamics. 

I can't say for sure, but my guess would be that there is a slight difference due to the intake tract on the two cars being different.  You'll notice for example that a car with a CAI has a different "calibration" than one without when buying aftermarket MAFs.  I would check and see, but my guess is that the sensors may carry different part numbers as a result, and if not the difference is likely in the MAF transfer function in the ECU meaning that there really wouldn't be any reason why you couldn't make that swap. 

This might be a case of "common knowledge", which is always common, but often not really knowledge.  Over the years the idea that you can't swap electronics into different sized housings may have been conflated into you just can't swap electronics.