|Combat table for Fighters DMG pg. 74|
|DMG pg. 82|
I don't really have an opinion on which way to go with the natural/adjusted rolls at this stage, my gut feeling is to allow adjusted results unless the target has some particular reason not to.
|DMG pg. 70|
|PHB pg. 38|
Now, look at the penalties in the To Hit Adjustments chart; in the same way as described above for "certain spells" these too are not modifications to attack rolls, they are modifications to the Armour Class of the target.
Consider a 1st level fighter with a battle axe vs an enemy in platemail and shield (AC 2). The fighter can hit AC 2 on an 18, but the weapon vs AC penalty of -3 if applied to the roll would make it impossible for him to hit; but, moving up the AC by 3 on the Combat Table results in a To Hit of 20; he can still do it. Contrast him with a level 1 fighter with a footman's flail; he too has a To Hit of 18, but he gets a +2 bonus and can now hit the enemy on a roll of 16. Both fighters can hit, but the one with the foresight to bring an alternate weapon reaps the reward. Similar trade offs can be made at other levels of AC.
Why is it so?
The paragraphs from the DMG pretty well explain the reasoning; it's about when and how a target should be "hit proof"; but, I would extend this explanation by comparison with later releases. 2e does away with the combat table for the much maligned THACO system, so effective was this change that many forget that THACO was not a 1e thing at all. 2e also introduced the rule that a roll of 1 is always a miss and a roll of 20 is always a hit (still the case in 5e). I think this dumbs things down and ignores the problem 1e was trying to deal with by making a broader threshold before a target is hit proof. 2e also simplified and made optional Weapon Type vs AC adjustment.
This combination of rule changes reduces player agency in weapon choice, and also, no minor thing, scales poorly in mass combat. In mass battle 40 Goblins with bows will, on average, hit a fighter with AC -10 (or whatever that is in ascending AC, yuck) twice a round under the simpler system and not at all in 1e.
The 1e solution to a "hit proof" target makes the threshold more difficult, but not impossible, to reach; which I think is superior to its successors. Like many rules in AD&D it might not be a totally accurate representation of the "real world", and you might go a long time without it being relevant; but, when the situation does come up the rules work well and do what they were intended to do.
If you've read this far you might be interested in what Nagora did to put all the numbers in one place on the character sheet.
The Combat Wheel
Of course, the ultimate instantiation of the Combat tables is in the Combat Wheel. I've lost track of where I sourced these from so I can't attribute them (but thanks to who ever it was). I sent these to a printer for laminating and it worked quite well.