Encoding is arbitrary. Morse code, for example, could have used any sequence of dots and dashes for any letter, and the only problem is making sure the receiver is using the same code. Physical processes, systems, events have no inherent meaning.
This is a problem for systems that require consciousness to arise in the brain as a result of the calculations its doing. How is the consciousness to know what it's supposed to be thinking of?
Going back to the Morse code, the machine that converts the dots and dashes to letters isn't applying 'meaning' to the beeps. It's a complicated lever, transmuting one encoding to another. Similarly, the physical parts of the brain do the same: convert one encoding (the sound wave a bear makes) to another (the electrical pattern encoding that sound wave). In purely reflexive systems, the encoding can trigger action without ever actually having a meaning - it's a complicated lever.
According to Modern science, the entire brain is entirely reflexive. There is no need to posit consciousness to explain its operation at all, so they think. It's simply not a player in the causal chain.
The ear doesn't know what it's heard. It's literally a drum and some resonant hairs. The audio cortex doesn't need to know what it's heard, it just does some transformations according to mechanical entailment - push lever A activates firing sequence B. What does consciousness interrogate to find out this is supposed to be a roar?
You could say the mind simply associates things, just like physical encodings are associated. It hears some random thing, and by experience, labels that as a roar. However, that is impossible. Mental representations are provably absolute, not arbitrary.
While the individual quale could vary in principle, the relationships between them are absolute, just like relationships between various encodings. Again with the Morse, dot-dot-dot is shorter than dash-dash-dash. It could mean A or Q or 5, or S and O as it does mean, but the fact O is longer to transmit than S is immutable. You would notice almost immediately if someone's qualia differed from yours. Sharp noises and sharp knives are sharp in a similar sort of way. Sometimes, folk with arbitrary qualia would feel like dull noises and sharp knives are similar instead.
There's a similar problem with accidental analogue computers. If computation itself lead to consciousness, then the fact every natural event is an analogue computer for some function would imply everything is conscious.