I've stumbled about the following statement:
An operation $b$ refines an operation $a$ correctly if and only if $a$ is more deterministic than $b$
As I would guess, it's exactly the other way around. Unfortunately, the wikipedia article about Z notation doesn't provide that information.
So, does operation refinement always mean, that an operation is made more deterministic or is it even possible to make it more non-deterministic?
Refinement is an operation on the abstract $\leftrightarrow$ concrete axis that moves a process/data description towards concreteness.
Read Top-down and bottom-up design in the context of Refinement (computing). Refinements replace (for data types) a less specific data specification with a more specific data specification. Refinements replace abstract program descriptions with concrete program implementations. In all cases, the degrees of freedom in the previous process description are reduced by constraint and specialization (e.g., "reification").