If you kill all the bears in Montana, or add a thousand more, the number of ants in Brazil doesn't change. They are not proportional, they're completely unrelated.
"having a constant ratio to another quantity"
Inversely proportional too is a mathematic relationship with specific meaning (for one the product must be a constant). It doesn't just mean "more pesticide, less ants" (that's a more general statement than inversely proportional).
Perhaps you merely meant correlated with respect to the rent. But even so, in this thread there were so many provisos added, that the relationship is nebulous at least.
If all else isn't equal, i.e. you change interest rates or tax rates, then you're changing the constant. But they're still proportional with the new constant. You're only changing the ratio, not the nature of the relationship.
That holds for everything. Only if they necessarily also move in the same way does this have any meaning, else it's like saying two points connect to a line.