Question on Rate Transition Block

Does anyone know why using Rate Transition block to change low sampling rate to high sampling rate signals has to use "Unit Delay" method? Why it cannot simply hold the previous value?

For example, input rate is 10 ms, output is 5 ms. Input updates at 0ms, 10ms... why output cannot be as: both 0ms and 5ms output value equal to 0 ms input, then 10ms get a new update, so both 10ms and 15ms value will equal to input at 10ms, and so on...

The current unit delay method shift the whole output later by one low sample period. I do not understand why Matlab is doing this.

Thanks.

Huan