Exact calculations in gr.interp_fir_filter_ccc

GNU Radio
1

I made the following simple example using the fir interpolation filter:
input sequence x=[1,2,3,4,5]

interpolation = 1

channel h=[1]
output sequence y=[1,2,3,4,5]

channel h=[1,0]
output sequence y=[1,2,3,4,5]

so far so good.

Now I changed to
interpolation = 2

channel h=[1]
output sequence y=[0,1,0,2,0,3,0,4,0,5]

channel h=[1,0]
output sequence y=[1,0,2,0,3,0,4,0,5,0]

why are the two last cases different?

thanks
Achilleas