Actually, I don't think it would have worked out quite the way you think (I may be totally wrong, so don't take my word on this, but...): Jan's base backstab multiplier should never go above x5, correct? Now, I don't think the game is able to tell the difference between a base backstab modifier and the actual modifier, so with the dagger equipped, the game thinks he has a base modifier of x6, which is wrong according to the information provided the engine. The chart says the modifier should be "x5", so the engine would endeavor to set Jan's base modifier to that value. But on the level-up screen, the game is confusing base vs. actual... It looks at the number in the chart, which is going to be the new value. It looks at the current number and sees that this is 1 point higher than what the base value is about to be set to, so it prints "-1" to the screen. But the fact is, the base value wouldn't be changed at all... it would be set to its current value. I'm feeling pretty certain that this is the case.
/edit
That was a terrible description, wasn't it?