No idea about .NET ... does the method implementation specify which interface it is for? But in Java the logic is fairly simple.
Interface A says the implementer must have method x(). Interface B says the implementer must have method x(). If you have method x() the syntax is good and you have satisfied the compiler's checking for both interfaces.
If the documentation for interface A says x() increments the x coordinate and the doc for B says it decrements the x coordinate, the semantics are inconsistent and some client of your class will be unhappy.
So there can be cases where implementing two interfaces with matching method signatures is possible, but not correct. If .NET matches the method implementation to the interface they might get around this issue.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi