Also, one thing I can't really understand, is how a resistor of a certain resistive material can have a tempco that can both be positive OR negative. The physics behind it isn't ambiguous, so why would the result be? All materials change resistivity by temperature, and it's usually well defined. So why can two resistors of the same material either increase or decrease in resistance by temperature?
Maybe because they are not the exact same material. Manufacturers try to minimize the tempco by creating special alloys/mixtures/whatever, but in real production, there are limits how uniform the alloy can be, hence some samples could have positive, some could have negative tempcos. The alloy could also drift slowly, but otherwise, the tempco is indeed either negative or positive, it doesn't change suddenly.
This being said, I think that the typical way they describe it (think about "+/- 50 ppm/degC") is just some sort of laziness; in reality, something like "(20 +/- 30 ppm) / degC" could be more accurate way to describe their actual sample population.
But indeed, it's very probable that two SMD resistors out of the same reel have tempcos very close to each other, and are unlikely to drift too far from each other in the same environment.