Cortex-M3 push instruction execution time difference by number of registers

When i execute "push {r1}", the execution time is 0.028 us.

then, when i execute "push {r1,r2}", the execution time is 0.042 us.

...

when i execute "push {r1,...,rn}", the execution time is 0.014*(n-1)+0.028 us.

I wonder why the execution time is doubled only when the first register value is stored in the stack.