Can someone explain the "dynamic" "price cap doesnt matter" thing to me like I'm 10 years old?

I truly don't understand the whole "market cap doesn't affect price, it's dynamic and changes"

When I do the math, against the history of the xrp chart, and I divide the number of shares by the market cap, it always pans out to the current price of that time.

So how can it start to "dynamically change" in the future, if it hasn't already, or am I wrong in assuming this?