Because that is how thought/belief and language works. It is that simple. If the cup is not on the table, then the statement 'The cup is on the table' is false. The cup cannot be both, on the table, and not on the table simultaneously. 'The cup is on the table' sets out the way things are, if the way things are is such that the cup is not on the table, then it does not hold good....just asking: why 'The cup is on the table' is a true statement IFF the cup is on the table?
Regarding that which is imperceptible with the naked eye, and therefore possibly unknown... One need not know anything at all about QM in order for that statement(the cup is on the table) to obtain. Nor does knowledge of QM change the states of affairs such that it negates the claim. The strange 'world' of QM is the same world as the one we observe with our eyes. No matter which electron is excited enough to emit a photon, the cup is still red. The issues that have arisen due to our knowing more about the subatomic 'world' simply do not affect/effect the macro world in such a way as to deny our knowledge of it.
On the matter of 'subjective' perception/interpretation... The cup is red, not because we perceive it to be, but rather the cup is red because the properties which belong to the cup reflect that particular bandwidth of the visible spectrum, while absorbing others, and we take an account of it. It would be reflecting that bandwidth(or not) even if we were not looking. The fact that it is also reflecting other, imperceptible bandwidths, does not negate the fact that it is reflecting the one which we can perceive. The fact that we are perceiving that bandwidth is what makes the claim 'The cup is red' true. Just like the case with 'The cup is on the table', the statement 'The cup is red', or any other statement for that matter, is true by virtue of it's obtaining universal states of affairs. Our knowledge of those affairs need not be complete in order for us to make true statements about them. A fact is a state of affairs that obtains in reality. A true statement is one that corresponds to reality/fact. Therefore, a fact would be the underlying metaphysical property of the universe and/or its contents in virtue of which a proposition has its truth value.
The opposing argument/skeptic will sometimes attempt to use the fact that we cannot perceive all that makes the cup what it is as ground for saying that our knowledge is incomplete. That is a true statement, our knowledge is incomplete. I grant the whole argument. Knowledge is accumulated. Therefore, it must be incomplete. Omniscience is not required for knowledge. If it were, we could never know anything. I know that I am typing a response to what constitutes, on my view, to be a rather confused set of counterarguments.
Regarding the 'aspect' of fluidity... As the cup physically changes, so too does it's appearance. As that change reaches perceptible proportions we take an account of it. As is demonstrated when the sun's rays hasten the deterioration of the cup in such a way that the molecules begin to reflect and absorb different bandwidths(the redness fades). As that occurs the redness of the cup, gradually, becomes less and less intense. It does not follow from the fact that we cannot perceive all that constitutes being the cup, or what the cup is becoming, that that which we can perceive does not obtain. While it is true that the cup always changes, it does not follow that we cannot make true claims about it. For if that were so, our knowledge of the cup changing could not be had.
My point in all of this is to make it very clear that an objection to truth/knowledge based upon the notion that our knowledge is incomplete is no objection worthy of much consideration. Nor does a claim to knowledge require immutibility or omniscience as the opposition would like to have us believe.
