If you recompile a class in a package, the compiler "assumes" classes in other packages have already been compiled. So any additions to a supplier class which your recompiled class has as a dependency may not be implemented in the .class file.
And if anybody else can find a more complicated or obscure way to explain it, I challenge you to try
He repaced his skull with glass. So you can see his brain. Kinda like this tiny ad: