I believe this is because Callable declares a return type, and Runnable does not.
From the JLS section 15.12.2.5, the overload with the most specific type is chosen, if there is one unambiguously most specific. This is what it says about most specific functional interface types:
A functional interface type S is more specific than a functional interface type T for an expression e if T is not a subtype of S and one of the following is true (where U1 ... Uk and R1 are the parameter types and return type of the function type of the capture of S, and V1 ... Vk and R2 are the parameter types and return type of the function type of T):
If e is an explicitly typed lambda expression (§15.27.1), then one of the following is true:
- R2 is void...
T is Runnable, S is Callable, Callable is more specific because its return type is not void, therefore Callable is chosen
Method overload resolution is very complicated, so there may be a bit I've missed, but I think this is why it chooses Callable
