Hi thatguy321.
First of all, I think the circuit network is the hardest part of the game to understand. So don't worry if you don't get it right away.
A chest with 50 iron plates is going to add 50 to whatever the value is for "iron plates."
If you have two chests and each chest has 50 iron plates, the circuit network will show 100 iron plates.
Any time any entity is connected to the circuit network, you can click on that entity to see how it relates to the circuit network.
By default, a roboport will export the number of items in the logistic network. So your first roboport sees that there are 50 transport belts and sends "50 transport belts" to the network, exactly as if it was a chest containing those items.
The second roboport does exactly the same thing, so it also sends "50 transport belts" to the circuit network. (When multiple items are connected to the circuit network they don't directly talk to each other. Instead, each one does its job in a vacuum and only talks to the circuit network.)
I hope this explains why the behavior is what it is. Each component has a job to do, and it blindly performs that job without regard to what anything else might be doing. Personally, I never have two roboports both exporting logistic information to the same circuit network. If for some reason I wanted to multiply the value by two I would use an arithmetic combinator rather than a roboport. But the great thing about the circuit network is that you get to be creative, and do crazy things like make tetris:
viewtopic.php?f=8&t=21647
There are a lot of great tutorials out there. (Do a quick search on Google or YouTube.) Or you can keep experimenting and come back here if you have questions. Personally I think the easiest things to understand are inserters, chests, and transport belts. Have fun!