The disappearing act of modifiers in GURPs

I have always had a sort of love-hate relationship with bell-curve rolls, like GURPS. For those who don’t know, GURPS uses a 3d6 as a base roll, which is then compared to your skill rank. If you roll lower than your skill rank, then you succeeded in your action. Now, I might talk later about how I tend to dislike roll under skill systems at a later date, but for now, i’ll try to stay on the topic of bell-curve and modifier.

Now, as anyone with some knowledge of statistics can tell you, a bell curve is à graphical representation of a statistic that makes, what else, a curve that spikes in the middle and gets lower at each end.  It is used to figure out probability of achieving a given result. When applied to RPGs, it means that you are much more likely to achieve a result of 10 or 11 when rolling 3d6 than getting an 18. GURPs, among others, work like that.

D&D and others tend to mostly use a  single die roll to resolve tests. Statistically, this makes a line; you are exactly as likely to roll a 1 or a 9, or a 20.

Now, at face value, a bell curve seems better, as you more likely to perform a given task averagely then you are to perform superbly. That makes sense. Your average performance is considered average because it happens significantly more often then the other results.

Now picture this, a thief is picking a lock. It's a darn good lock so it affords a -2 penalty. In a d20 system,  it's a flat 10% penalty it doesn't matter how.good a thief you are, it's 10% and that's it. Now for a roll under, Bell curve system, it gets pretty funky to calculate.  Since it's a roll under system, you succeed if your roll is lower then your modified skill rank. But since it's also a bell curve, that means each +1 has a different % value, depending on the skill of the thief. So a thief with a skill rank of 14 who gets hit with a -2 penalty finds his chances success lowered by roughly 16%. Now, a same thief with a skill 16, picking the same lock, only finds his chance of success lowered by about 10%.

WTF. Bonus and penalty, applied to different people, have different value. That makes no sense to me. If lock B is twice as hard to open as lock A, then it should not be 100% more difficult to open for an average thief, but only 75% harder to open for a skilled thief. Characters with weaker skill and then double boned. They are less chance to actually succeed normally, and are hurt more by the same amount of modifiers.

I understand that a more skilled character can succeed more often than a less skilled one. That is normal, and expected. But a more skilled character has a higher skill score, meaning it automatically compensate for penalty for doing harder stuff. And something that is hard, should not be less hard because someone is skilled. The penalty should be worth the same amount of % chance of failure.

Now as to how to fix that, I haven’t got a clue, but I’m thinking about it!