You are free to believe whatever you want about how YouTube works, but don't come here with misinformation. YouTube doesn't take "around" 45%, they take exactly 45%.
When a video playback occurs, you have the potential to display advertisements. Now, YouTube has multiple methods of displaying ads, which means that you can register more than one impression per video view. You could have an instream unskippable video ad, but you could also have maybe an overlay ad or sidebar ad as well.
In playback based CPM, all of the ads on a particular playback are combined into one value for that playback because that playback's value is determined by the total ad value spent. For impression based CPM, the value is individualized based on each separate impression and your value is averaged over the total number of impressions, rather than the total number of playbacks with impressions combined.
This results in ... *Mathematical Drum Roll*... A lower value for Impression Based CPM than for Playback based CPM. They represent the same essential subset of data, but merely display it in a different way. How much were 1000 impressions worth on average, or how much were 1000 playbacks which can have multiple impressions each worth on average. Not to mention that the vast majority of impressions everyone gets are the lower paying overlay style ads which will also drag the average CPM down and will impact Impression Based far more than Playback based due to the method of averaging the values.
I'm not shooting the messenger. I'm shooting the fool arguing who hasn't taken the time to bother to read anything on what he's arguing about.