Why can ad-fills be significantly lower than ad-requests?

The primary factor is the fill-rate of your ad-demand sources. Most ad networks have much lower demands than publishers would wish for. This is simply due to the fierce competition between a multitude of old and new ad networks appearing on the scene, and the fact that the biggest, most established ad networks, sometimes connected to major social media sites, attract the majority of advertisers, leaving other ad networks competing on the remaining advertiser accounts. 

Another factor is the request volume. It requires ad requests well above 1 Million per month continuously, in order to receive higher priority with ad networks, higher fill-rates and better CPMs. This means: apps and websites with large traffic receive disproportionally better fill-rates and CPMs, than apps and websites with monthly traffic numbers below 7-figures or multiple 7-figures.

And last but not least: OTT/ConnectedTV devices and players often pre-load ad requests for mid-roll ad-breaks when a video is loaded, which means that, many unfilled ad-requests are generated by the device / player, which will never be filled, whenever viewers do not watch the entire video, but exit or jump to another video before arriving at mid-roll breaks. Some OTT/ConnectedTV devices do this in order to call the demand sources (ad networks / exchanges) at the beginning of the video along with the pre-roll ad-request, in order to increase the chance, for mid-roll breaks to be filled. This means, that the mid-roll ad requests were loaded in the beginning of the video, but were never filled because the video was not played long enough.