In some situations it is not possible to place an ad on your video. For example if someone skips a pre-roll ad before it has completed then this would cause it to count as a monetizeable view which was not monetized. This is because it was selected as appropriate to monetize but because the action could not be completed (viewer skipped the ad) it was not monetized.
Another example of why its not possible to monetize all views is when viewers use ad-blocking software on their browser, which would prevent any ads being shown.
Also, some countries are not appealing to advertisers so viewers from those countries may not see any ads even if a video is being monetized. Views coming from specific countries will not be monetized. To increase your monetized views, we suggest creating content that is attractive to viewers in countries where advertisers would want to show their ads, for example count