You are comparing apples with oranges with those analogies

A Traveller character with +1 is the equivalent of a 3rd level fighter
A Traveller character with a +3 is an 8th level fighter.
Put them in identical armour and give them identical weapons, the 8th level fighter is going to win the day. This is true in Traveller and D&D. A lot of people don't follow the statistics of DM bonuses to a 2d6 probability range. It should really be printed on every referee screen and character sheet
From LBB:0
| Target no. | 2+ | 3+ | 4+ | 5+ | 6+ | 7+ | 8+ | 9+ | 10+ | 11+ | 12 |
| probability/% | 100 | 97 | 92 | 83 | 72 | 58 | 42 | 28 | 17 | 8 | 3 |
To get it into similar territory make the standard target number a 13 on a d20, The 3rd level fighter (Traveller skill level 1) gets a +3, the 8th level fighter (Traveller skill level 3) gets a +8.
Give them both standard armour but with damage reduction (there is a D&D variant for this) and give them both the same weapon.
the +3 skill/8th level fighter has a considerable advantage in both systems. Yes the +1skill/3rd level fighter can get lucky, but the probability favours the higher skill (higher level) fighter.
Try it out.
777777 combat skill 1 vs 777777 combat skill level 3
Run 1,000 combats, Which character wins most often by a considerable margin - all down to +3 having a massive affect on a 2d6 spread with a target of 8+. It gets worse at +4 and +5... the 2d6 system really can't cope with much more than a capped +3 bonus, you have to resort to raising target numbers until you go beyond the maximum possible on 2d.
Much as I love CT, 2d6 and all the rest of its idiosyncrasies I am aware of its limitations. Increasing the dice to d8s, d10s or d12 have interesting effects on the game resolution. Similarly increasing the die type as a bonus damage system is something that can be explored - but that way lies heresy
