One affects the other is why. If you hide, and are not moving it is very easy to be hidden, now try and move and remain that quite and hidden. That is why there are two checks, are you hidden, and how well did you move. A failure in either would mean that the person heard you coming and could ready themselves for your attack.
That list is from the AD&D manual if I rember correctly.
Pick Pockets - 15% how you steal something from some one
Open Locks - 10% picking the lock
Find/Remove traps - 5% disarming the trap (hey we get to be sappers/enginers)
Move Silently - 10% how well you move and stay undetected
Hide in Shadows - 5% how well you hid
Detect Noise - 15% this was for detecting other hidden npc
Climb Walls - 60% climbing up stuff
Read Languages - 0% a thief eventually at level 10 could gain the ability to read magic scrolls and to cast from scroll this affected how they could do that, and if they could figure out codes and languages of new people/races
They kinda had the thief doing all the spy/rogue/assasin/stealth/scout thing.
Hope that helps, and yes it probably does cause some lag.
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Drink and be merry
Jimbo
"Renegade, Rebel, and Rogue"