return value of execute(command) when command has "silent!"

Question

In my understanding, the return value of execute(command) should be what command outputs. (The second parameter of execute is to affect whether this output will also be displayed in vim.)

Now command runs a function and inside the function there is silent! echo 3. so command itself does not have output. Shouldn't the result of execute be empty?

Ex:

function! s:x()
  silent! echo 3
endfunction
let x = execute('call s:x()') 
" let x = execute('call s:x()', '')  does not alter the output
" this means `call s:x()` itself has no output
echo 'x is' x

shows

x is
3

instead of

x is    

Is this a bug? How to explain the phenomenon in the comment?