Agree that the total is not particularly meaningful. However, I think the rate of increase corresponds to the rate at which happiness declines. (That is, more visitors coming in correlates with happiness dropping more slowly.)
My best guess of the chain of causation is something like park "activity" (including number of attractions and decorations out)

slower decline in happiness

increased rate of visits. So the running total is ultimately a reflection of the sustained level of activity in the park - an effect rather than a cause.
I don't have any hard evidence of this - just my impressions from observations over time. Once I have my second game fully leveled up I have some experiments in mind to confirm though. Would love to hear if anyone else has any more concrete info already, though!