This article covers why you could be seeing very low Edge Cache reduction.
Edge Cache reduction depends on how much web content is served from the cache versus retrieving it from the internet.
If Edge Cache cannot find a requested object in the cache the request is made to the internet to retrieve the object.
When you observe that the reduction is very low it just means during those times we are not able to find the requested objects in the cache, so getting them from the internet.
This depends on what users are accessing at particular times.
Leave the Edge Cache on and you will see improvements in the reduction as the cache grows and as users access the same content from the web.