The game where I was told this was necessary was back in 1.41. While I was testing this in 1.49, I finally twigged on what you meant in your deconstruct analogy, and discovered I was partly wrong.
Two examples from my testing:
Race A has Ship Construction 2, Projectile Weapons 4.
Race B has Ship Construction 3, Projectile Weapons 1.
If A proposes
Give:
Projectile Weapons
Want:
Ship Construction
and B accepts, A gets Ship Construction 3 and B gets Projectile Weapons 2. (This is the part I thought contradicted the manual, but when I couldn't find where I'd read what I thought it said, I realized I was thinking of another game entirely.
)
If A wants to give B Projectile Weapons 4, he has to offer
Give:
Projectile Weapons
Projectile Weapons
Projectile Weapons