a really long, detailed explanation for how just about every stat is converted to percentage, and what it all means.
The really short answer to your answer is that, physical mastery, and basically any other stat goes through an equation, and part of that equation is being divided by your level. So as you level up, you need a higher rating to maintain the same damage bonus.
The good news in all this is that itemization has been at least slightly modeled around this, with items at each level cap being slightly more powerful relative to your character level than those of a previous level cap. While this generally holds true, since its unlikely you've decked yourself out in rift/Helegrod raid gear at lvl 50, and are probably equipping purple/yellow crafted and quest rewards items, the damage bonus works out a little more hit or miss, depending on whether a quest chain you did had good rewards for you or not.
[center]Let our actions speak for themselves. Jinjaah has been pouring over every post in the Bullroarer forum. Please keep in mind that any experiences with previous LOTRO teams are not reflective of the current team, give us a chance[/center]