Hearing is logarithmic, the response is non-linear, ergo you may hear X000Hz but not hear X001Hz, yet hear X002Hz. X003Hz may be easier to hear, yet X004Hz may

Hearing is logarithmic, the response is non-linear, ergo you may hear X000Hz but not hear X001Hz, yet hear X002Hz. X003Hz may be easier to hear, yet X004Hz may be more difficult than some. It's not a smooth response. Take a look at some hearing response graphs to get an idea of how variable it can be across the frequency spectrum.