Starbase trade ports

So, the infocard for the Trade Port starbase upgrade says that you get 1.3 credits per second.  TEC get a second level that produces 3.0 credits per second.  I figured this didn't sound like much of a boost, so I mostly was using the trade port to connect empty spots in my trade network, and I wasn't buying the second level at all.

But the other day, I was reading over the trade income report, and I noticed that a wormhole gravity well was producing a lot of trade income -- 3, maybe even 4 credits per second.  "That can't be right", I thought, "I know I only purchased the first level of upgrade."  So I tried installing two trade port upgrades on a non-front-line starbase, just to see what would happen.  It increased that planet's income to about 14 credits per second!    OK, that's more like a 12/second upgrade (since it already had one trade port), but still pretty nice!

So, I'm liking trade port slots a lot more now, but I have to ask: how is this number calculated, and what's its relationship to the number shown on my infocard?  I suppose that I get a boost from the longest trade route (was about 5 hops), and I had a 10-20% boost in trade income from research (as I recall), but can I really get 12 credits per second from that?  I also don't understand why the increase seemed to be much more than a factor of 2, but maybe I've just got the wrong numbers.