According to CanIUse, border-radius
is supported by the Android 2.2 browser. 2.1 requires a prefix but is also supported. Therefore, Modernizr is reporting correctly.
However CanIUse also notes that Android 2.3 (and presumably earlier) does not support percentage values for the property. That'll be why it's not working for you.
I guess Modernizr is looking at basic feature support (which it has). You may need to write your own more specific test for this particular case.