> But a person on ISS will tell you the same thing about earth!

You can only say that about spaceships flying past Earth in a straight line. The ISS goes in circles, i.e. always changes directions. Only the Earth reference frame is "inertial". As Duros001 said above, this creates not just a perceptual but an actual measurable difference between the clocks on Earth and ISS - the clock on ISS will tick slower and all observers will agree on that.