[Andrs]

If H is the union of H1 and H2 and assuming that H1 differs from H2, the hypothesis set H should be at least as complex as H2 therfore it should have a lower deterministic noise.
Related to the question:
The basic charachteristic is that H' is a subset of H therefore H' contains fewer alternatives compared to H (less options less flexibility). That is, H' will not be able to fit into the target function f as well as H would have. That is, H' implies a higher "deterministic noise".