Reliability of a service

Question

Assume that I need to determine the reliability of a service. The service includes component a (software reliability=0.95) and component b (software reliability=0.98). I have 2 computers: Computer A (hardware reliability=0.99) and Computer B (hardware reliability=0.99).

I have two following cases:

Case 1: Deploy both a and b on computer A. For this case, the service reliability is around 0.923.

Case 2: Deploy a on computer A, and b on computer B. For this case, the service reliability is around 0.912

I really wonder why the service reliability in case 2 is lower than in case 1. The thing is A and B have the same hardware reliability. Can someone clarify that?