OK, so, we have speaker A with a ruler flat frequency response, but speaker A only reproduces frequencies between 5000Hz and 10000Hz; we have speaker B with a ruler flat frequency response, but speaker B reproduces frequencies between 20Hz and 20000Hz. The FR curve is indistinguishable.
But they aren't indistinguishable. The portion between 5-10KHz may be, but that doesn't make the entire curve indistinguishable. That's like having one curve appear smoother than another...but only because the "smoother" curve uses significantly larger dB increments in the Y axis.