Internet data transmission are asymmetric in nature, meaning you send a small portion and receives a download which is much larger than the one you send, I am not very sure about, task managers ability to determine actual traffic, but I assume it was averaging total traffic, which is what, probably is happening.
Your connection to a service provider is a shared resource, meaning that the total capacity of the provider are shared among subscribers, however that is not constant there will be some time when the connection and traffic permits you can have the full available bandwidth, keep in mind that the allocation of xMbps are not entirely dedicated to data traffic, it also contains redundancies and traffic control data and some of that may be lost during transmission, and thus will occupy another space during next transmission, which by the way may have another value for the resources.
Many Internet subscriber share your predicament, sadly there is no device that will make your experience any better.
I can't say why. I can only say you are lucky to have such. You should be grateful for it. Here in my country, it's rare to even have enough data to do what you want. There are no free wifi and data subscription is expensive. I spend four times what I should normally spend every month because of Steemit and some other things I do on the internet. Even if it were just streaming music, it would still sap my data. Our data providers here aren't as kind as yours are and makes us pay exorbitant fees for the little amount of data they give us with every subscription.