What does the CPM value inside a push subscription traffic monetization platform relate to?

[VastlySeeds]

What does the CPM value inside a push subscription traffic monetization platform relate to?

I found that for about the same number of impressions the CPM value is sometimes high and sometimes low, I asked customer service and AM and their replies were very vague.
If I want to increase the CPM value the advice AM gave was to give more traffic, but I do want to know something about the details.
@PropellerAds @Roller Ads @PartnersHouse @TacoLoco @Monetizer