Originally Posted by T.G. Oskar
Basically. Pathfinder does the same as well.
That said, it's a strange coincidence that PA is always limited to a +5 bonus to damage (+10 with 2-handed weapons) instead of how the system works. It leads me to think that the intention of the developers was to limit PA to +5, just like with Combat Expertise (which reminds me: what would have happened if Combat Expertise was similar to PA, with the same bonus if fighting with a weapon in one hand and nothing in the other?). I mean, even Pathfinder went with the idea, and that's a successor system.
Limiting a number to 5 or 10 is no coincidence at all. There are a number of plausible explanation for doing this.
1. They may have been copying one another. Power Attack is a pretty classic feat and possibly the most common feat ever taken. The idea that one group of people making a D&D video game would copy another group of people making a video game isn't surprising at all.
2. Also, there are effectively 20 levels in D&D and splitting that number up into half (10) or one fourth (5) gets you those same numbers.
3. Giving a person a penalty of -5 to attack isn't a huge surprise and won't be too painful. A first level melee character like a Fighter or Barbarian is likely to have around 18 strength and +1 BAB. That's +5 to attack, which is also the same number as the common video game Power Attack penalty.
4. The penalty to attack from Combat Expertise is -5. This is a feat that deserves the limitation since a character with high enough AC can get to the point where they're only hit 1/20 times. A party with a character of AC 40 and three of AC 20 creates an odd and difficult combination. This will be even more problematic if you're designing a computer game that isn't going to be modifying itself to artificially increase or decrease attack rolls based on the PC's AC.
5. Last and likely most relevant is the fact that we have 5 fingers and 2 hands. This may sound like a stupid reason to you, but its the real reason why we have a Base 10 math system. If everyone had 3 hands and 6 fingers on each, we would likely have a Base 18 math system and Power Attack would give a -6 penalty to attack. It's also the reason why our calendar was based on 10 months (before Caesar and Augustus changed it) and the reason for all sorts of other things in our lives.
Five is a very common number and it should be of no surprise to anyone that it would get picked out for use in almost anything.