An orc getting +2 Strength means if you take a random orc and a random human out of a thousand, the orc is more likely to be the stronger one. One would therefore think the strongest orc would be stronger than the strongest human, and yet their Strength scores share the same cap of 20. Why? Because the ability score system is arbitrary that way. It's a game mechanic.
I think it has something to do with the fact that despite being practically the same height, both a human and an orc can only reach a certain muscle density. It is simply physically impossible for a normal humanoid race to naturally have a strength higher than 20 without relying on magic, alchemy, or gods to do so.
But even if you put an orc with strength 20 and a halfling with strength 20 next to each other, the orc will be stronger. Why? Because his weight will be significantly greater and will give his punches and grabs extra power.
The same thing is true of dexterity. Even with identical stats, a halfling will always be faster than an orc, because due to his weight, the gravitational force will work on him with less intensity, allowing him to move faster.
