Why explicit interface implementation works that way?

The other answers do not correctly identify the C# feature that you have stumbled upon.

You have discovered a somewhat confusing feature of C# called "interface re-implementation". The rule is that when a derived class specifically re-states an interface that is already implemented by the base class, then the compiler starts over and re-does the interface mapping from scratch.

If that happens then methods of more derived types are given precedence over methods of less derived types because we assume that the developer who is developing the more derived type has a better implementation than the developer who developed the base class version. After all, if the derived version was worse, the developer would not have implemented it!

This rule allows you to decide whether you want a derived class to replace a base class interface mapping or not, because sometimes you want to, and sometimes you don't.

See my 2011 article about this feature for more details:

https://blogs.msdn.microsoft.com/ericlippert/2011/12/08/so-many-interfaces-part-two/

You might also find this answer helpful:

Abstract base classes that implement an interface

For the section of the specification that describes this language feature, see

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/interfaces#interface-re-implementation