Noise?

In my sister's recently-moved-in-apartment, strange noise is often heard from the upper floor. At first, I guess it's the noise produced by the water pipes, but it's quite different from what I heard in other places. My sister, using her unique intuition, suggests that the noise is the sound of a walking dog! OK. The noise really sounds like the pace of a dog. But if we can hear the footsteps of a dog, how come we cannot hear the footsteps of the residents? Weird!