Why does the other team always add when we give them D for forwards, and vice versa?
Short answer is they don't. It's a valuation thing really. Get someone from the other team in here and they'll tell you the add should be the opposite direction. We're all ballparking offers here, the only people that can really assess value accurately are GMs of actual teams that know what they're willing to part with to gain a particular asset, whether it's Trouba from us, Nylander from the Leafs or anyone else.
I do believe there are people on here that can reasonably accurately guesstimate what returns for particular players should be. I'm not one of them. I just try to absorb what the people I believe are more knowledgeable than I (assessment based off being on the site for years now and seeing/remembering who has better accuracy) put out there and adapt it to my own opinions to try and temper them.
In the above example you quoted from me I didn't list specifics but if I did I would have put something like this:
Trouba returns: 1st rounder (15-20 range), A to A+ prospect that is close to ready for the show (next year in limited or full action and contributing) and B prospect (think Petan like, not necessarily much use but potential or AHL addition).
One of our forwards: I was thinking Connor (from the rationale of cap management) or you can substitute Ehlers to Carolina because they have a decent assortment of RHD (Trouba replacement). Connor should have huge trade value in that he'll be a kid coming off his ELC with a pair of 30 goal seasons. He's not going to be cheap but he is very good and goal scorers are desirable. Doubtful Carolina gives up their #1 RHD so we're getting a 2nd pairing RHD, hopefully someone youngish able to handle 1st pairing minutes with Morrissey as a partner. But that's a lesser return so they add a pick or a prospect we want.