I think its been talked about how the number/type/strength of Pokemon that appear is based off the number of active players in that area. More people that play in the area, the more likely you are to catch strong, rare pokemon. Im currently at work, but I will try to find the article I read that from.
As for the Amusement Park factor: Everywhere is using Pokemon Go to help bring people into the park as an added incentive it seems. I think Ive seen at least 15 or so links just from Parks alone giving out a full rundown of where the stops are to what you should/shouldnt be doing while playing the game. I stated it elsewhere, but this game may bring in more visitors to parks/attractions that wouldnt normally go there. The family of 6 with only a couple of thrill seekers has something else to do inside these parks, and may spend longer times within the park with potential revisits.
As for myself, the best I got is a Pinsir, a Pidgeotto and a Paras/Growlithe of above 300, but below 500 in CP that would do piss-all to the gyms around me with Mon around the 1k+ range in Power.