A predictable environment is an environment for which an agent has an adequate (or perhaps complete) world model. For example, an agent that had a sophisticated, first-principles model of Newtonian physics could predict with reasonable accuracy the results of throwing, with a known force, objects of known mass. However, since such models are computationally prohibitive, most agents consider a dynamic world unpredictable as well.
This assumption does not hold for agent's that behave in simulated, dynamic worlds. Since those worlds can generally be predicted exactly (e.g., a grasp command always results in holding an object if the object is holdable and the agent is in the object's proximity), these dynamic environments can be considered to be predictable.