How does a 160cp Chikorita break out if six balls with berries?

Catching the starters, and Pikachu suck...

Let me know when its 10&10 and then I'll sympathise.
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I am certain that there is an additional metric in each pokemon that determines escape rates not seen in the circle color. There are always like 1/30 pokemon that burn WAY more balls than is reasonably expectable.

The base catch rate of Chikorita is only 20%, so even with berries and great throws, there is a reasonable chance of it breaking out.

Each gen's starters have always been difficult to catch. Besides their base forms, all their evolutions are difficult too. I burned through a lot of balls and berries when trying to catch Charizard, Blastoise, Venusaur, Typhlosion, Feraligator, Meganium... a lot more than I need to catch Snorlax, Lapras, Tyranitar