>Approximately, the probability of catching a Pokémon starts with the species' catch rate divided by 255. Then it is multiplied by the following factors
The health of the Pokémon (relative to its full health), which can reduce the probability to ⅓× at full health.
The type of Poké Ball, which can augment by some ×.
Any status ailment of the wild Pokémon, i.e. 2× increase for sleep or freeze, and 1.5× for others
The reason being is that they have a formula, and Pokemon that can't be found in the wild can apply to this formula as well.
You can also ask Game Freak.
Hope This Helped :D