Lots of factors are involved but it boils down to supply and demand for the ad space. Some niches are very lucrative for the advertiser and the value to them for getting a new customer is very high and so they are prepared to bid very high rates per click. An example would be credit card companies. A new customer would probably be worth thousands of dollars to them so they would be very interested in advertising on videos related to credit.
Other factors are:
Time of year (end of year > beginning of year)
Target demographics of channel. ( A channel whose main audience is in India would probably earn less than a channel whose audience is in the USA, for example.)