Think of it like an unbounded array of integer temperatures values -- if you keep adding memory you can keep adding more positive values to the end of the array infinitely, each with an indexable location (i.e. enumerating the set). But this concept breaks if we just say we'll throw all the negative values on after infinity; if we're infinitely adding positive values to the array, we'll never have the chance to stop and start with those negative Kelvin values on after. When this happens it's called non-enumerable (or more formally fails the test of diagonalization). It seems that the authors of this system chose to make +infinity an arbitrary enumarable point to show the negative Kelvin values in excess of that point. Having said that, the set of all integers (negative, 0, positive) should be enumerable (because you can just *=-1 each index), but not when infinity is a member of that set. I think there's a numberphile and veratisium on why not all infinity's are equal if curious to explore.