You could have tinnitus at a frequency you can't hear if the tinnitus arises downstream of auditory issues that reduce hearing sensitivity.
For instance, if someone couldn't hear a certain frequency because the corresponding hair cells in their ears were damaged, they could still have tinnitus at that frequency arising from parts of the brain that process that frequency.
They can hear the tinnitus, but not real sounds at that freq.
IANAD, is there any kind of "gain correction" theory for tinnitus? ie, the brain notices your ear can't hear this frequency very well, so it pumps up the volume ... maybe too much.
That would fit this data point, at least.
Weirdly, maybe not so different than when a hearing aid starts ringing.
Go back to the old question of if a tree falls in a forest, does it make a sound if no one hears it?
Tinnitus isn't caused by actual waves of air pressure on the eardrum. We'd not call that tinnitus (unless it was actually inescapable, eg. a perception of rushing blood flow), that's just an annoying real-world sound.
Hearing is connected to eardrum vibrations by a complicated system of the outer ear, eardrum, tiny auditory bones, the fluid-filled cochlea, tiny hair cells with sensory neurons, the auditory nerve, and eventually the brain.
When all these work properly, the brain gets accurate information about sound waves.
When those hair cells are damaged (or wherever the stimulus arises) to send bogus information up the auditory nerve, the audio perceived by the brain as a result of that stimulus is not necessarily correlated to any accurate frequency data that could make it through that pathway.
Since tinnitus uses different mechanics and pathways, that seems plausible.
For comparison, think of a recently blinded person still dreaming visually. Or think of phantom pain.