The type of cap limits the available tolerance. And aluminum electrolytic is rarely tighter than ±20% and more likely will be listed as +80/-20% because they're used more for bulk capacitance (the more the better) in power supply filtering. You have to have stable dielectrics to achieve the ability to gain tolerances of ±1%. Polyester, polypropylene, polystyrene, silver mica and such can achieve those. Caps are usually used against resistors for frequency or timing, so standard values are limited to the 20% tolerance category, even though the tolerance may be a lot tigher for the component. It's usually the resistor that's selected or made variable after the basic capacitor is selected.