It's a common misconception but it's a little more complicated than that. A CPM isn't what you get paid per 1000 views of your video, it's what the advertisers pay per 1000 views of their ad.
If an ad runs on your video - and 999 other videos - Google charges that advertiser an agreed upon sum for showing the ad. Because you accounted for one of the one thousand ad views, you get 1/1000th of that money, minus google's take which is a commission on selling and serving the ad.
This happens hundreds, thousands, hundreds of thousands or (if you're lucky) millions of times on your videos and channel. One ad on your channel may pay a $4 CPM, one a $3.50 CPM, another an $18 CPM, another an $8.17 CPM, and so forth and so on and on and on and on and each time you get 1/1000th of that money.
YT then does the work of tracking how those ads are served on your channel, what all the CPMs were for each, what your cut of those ads is, and then adds it all together and pays you for it, while telling you what the *average* CPM you got paid for is.
Additionally, the CPMs the advertisers pay (even on the same ad) will vary depending on the time of day the ad is served, geographic location of the viewer, demographics of the viewer (age, income level, sex, etc), the industry the ad targets, the level of competition for the keywords the ad targets, the perceived premium quality (if any) of the video/channel the ad is running against, and a bunch of other factors.
When someone says they can get you a higher CPM, what they're actually saying is they can put your video inventory into a pool of advertisements that pay a higher CPM rate.
The only time a CPM is what *you* get paid per 1000 views is if your videos serve the exact same ad, at the exact same CPM, 1000 times, giving you 1000/1000ths of the price the advertiser pays.
