Different niches pays more and others less, there can be a huge difference between niches Picking a higher paying niche can get your CPM up. But of course the high paying niche could be very competitive making you less money since you get less views with the tougher competition.
Your viewers location, there is a big difference on how much the ads cost in different countries. As an example right now according to my analytics I get a CPM of $13.37 from my viewers in Switzerland and $8.95 from my viewers in the US but only $1.33 from my viewers in Thailand and $0.86 from my viewers in Argentina. So targeting your videos for viewers in high paying nation can have a large effect on your CPM if they end up watching your content.