Usually what happens is upon listing, some coins have a lot of hype behind them which will result in the price surge of the token which will give them a higher rank and market cap. After a couple of days when the hype slowly cools down the price drops which lowers their market cap and the rank.
In simply terms, these coins are manipulated into those top ranks so as to get the attention of noobs or gullible Investors hoping to cash in on the hype surrounding that particular project. Exchanges are on, on this scheme too as they play major roles in making these projects look legit meanwhile they're pump and dump coins created with the mindset of defrauding Investors of their bitcoins.
Development like this proves the old saying, you shouldn't rely on others for investment advice. Most individuals uses this ranking sites to influence their investment but that shouldn't be so.