Yeah, I just used Sneak/Alertness because I'm more familiar with them. Also, I see what you're getting at -- with a higher level, fewer people will succeed against you.
But no matter how high a level you get, *someone* is still going to succeed against you, once every five throws or so. Maybe that's the intent behind the system -- no one should ever win all the time, and no one should ever get so good that the small fry still can't land a hit every now and again.
If that's not the case, though -- if the challenges for Larry should be less than mere annoyances to the superior Joe-Bob -- then perhaps the system should reflect that differently.
Here's an example. I can't say if it's an improvement; it's just different. Maybe this has already been tried and rejected. I don't know.
On a 1: Challenger subtracts five from total.
On a 2: Challenger subtracts two from total.
On a 3: Challenger doesn't modify total.
On a 4: Challenger adds two to total.
On a 5: Challenger adds five to total.
Now let's look at the second example again. Joe-Bob's defense is 10 (theoretical max). Larry's is 4 (still pretty new to this). The thugs are at 4.
Joe-Bob and Larry both throw a 2
Thug 1 throws a 1 totaling in 4
Thug 2 throws a 2 totaling in 6
Thug 3 throws a 3 totalling in 9
Thug 4 throws a 4 totalling in -1
Thug 5 throws a 5 totaling in 2
In this setup, the focus is less on the randomness of the chops and more on the skill of the individuals. There's no way any of the thugs would be able to spot Joe-Bob, because they're ordinary mooks, and he's awesome. However, they're nearly on-par with Larry, which is why two out of the five see him. If challengers won on ties, it would be three thugs who saw him, so essentially we're looking at the closest to a 50/50 comparison as you can get.
Now suppose that Uber-Thug stepped in. His third eye of WTFulu lights up and he starts looking around with 10 ranks of bad-juju-alertness-powerz. Larry stands no chance at escaping the notice of this freak, but Joe-Bob's on par with this mofo, meaning there's a three-in five chance of escaping his notice (two-in-five, if Uber-Thug had eleven ranks).
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For the tldr version, I'm suggesting that auto-wins and losses have some limitations to them (which may be a good thing, depending on the system's intent). Personally, I feel that The Little Guy should never be able to hit The Really Big Boss...certainly not once every five shots. The adjustment as above makes the superior (or inferior) talents of individuals shine through more clearly. As it stands right now, five starting PCs could jump in The Boss Fight, whip out shotguns, and at least one of them would hit.