Regional Difficulty?
Contrary to what some people believe, the difficulty level in Resident Evil 5 does not vary from region to region. For example, the European version is not harder or easier than the American version. The game is exactly the same, no matter what region, no matter what platform.

Think about it for a minute. If the difficulty level did indeed vary, then how could people with the easier version get their scores on the same leaderboards as everyone else? Wouldn't they have their own, seperate leaderboards?

In Game Difficulty
We know there are four selectable difficulty modes in the main game, but there are also eleven invisible grades of difficulty, ranging from 0 to 10. These grades vary, depending on how good (or bad) you are playing:

                                                 Starting
                               Grades     Difficulty
       For Amateur: 0 to 3              1
         For Normal: 2 to 7              3
        For Veteran: 7 to 9              7
For Professional: 10 (fixed)      10

When you start a new game at normal difficulty mode, for example, you will start at grade 3. If you keep playing badly (take damage), the grade will eventually drop to 2. In other words, things get slightly easier.

If you play really good (i.e. avoid damage and kill enemies) the grade will slowly creep up. So you could get to grade 7 at normal difficulty. Grade 7 is the starting difficuly for Veteran mode, so you are effectively playing at Veteran difficulty mode, even though you selected normal difficulty mode.

That's the thing about this game, difficulty varies throughout the game and difficulty modes can overlap.

There are some exceptions. Professional mode is fixed at grade 10. When you play 'The Mercenaries', the difficulty remains fixed. It's grade 6 for Solo mode and grade 8 for duo/split screen mode.