This is expected behavior only. The execution depends on step size used to simulate the block, change the solver step size in configuration settings:
Solver option -> Type: Fixed-step
Fixed step-size: change from auto to 1
You will get output as you expected.
In your case it looks like solver type is variable-step in which solver adjusts step size to compute for results so the behavior is like that.
For more details on how block works you can check this link: http://in.mathworks.com/help/simulink/slref/repeatingsequence.html#bshfqei
