How come there is such a big difference with and without the cache function?

Can someone please explain the big difference between having the cache feature activated (and a cache plugin) and not having a cache plugin and not have the function activated in WP Statistics?

I get it (I think) that a lot of automated boots and search engines are filtered out but what I do not get is why this is done with the cache feature activated?

There should not be this big difference between having the cache feature activated and not having the cache feature activated – should it?

In my case there is a difference around 35-45%. I get that amount more visitors, daily without a cache and without having the function actived in WP Statistics. I am not that concerned about the numbers, just curious as of why there is such a big difference?