The larger the data sample the more accurate the percentage.

If you have 20% parry rate with one mob hitting you over the course of a minute the RNG will have a broad range.

If you have 10 mobs hitting you the range will be more narrow and closer to the 20% mark.

The more predictable parry is the more useful it is since it's biggest flaw is how unreliable it is.

(This still doesn't make it good but outlines why it is more useful during a duration with more mobs)