I want to point out that this glyph is actually better than that math makes it look.
Let's assume the 2000 base armor and Improved Inner Fire as a before case. This gives you 3538 bonus armor for a total of 5538 armor and a physical damage reduction of 26.66%. With both the glyph and Improved, you get a total armor of 7307 and physical damage reduction of 32.42%. Thus, the total damage reduction difference is 5.76%.
Now, just looking at that number, you might expect that a hit that would hit you for 1000 without the glyph would hit you for 942 after, but that's not the case because the relative difference is actually higher. The relative damage reduction is higher and, thus, a hit that would have hit for 1000 without the glyph would hit for 922 afterward. IOW, it's a relative physical damage reduction of 7.85% based on these assumptions.
There's a couple ways to look at it. The best way is this: A hit for 1,000 with 26.66% reduction was actually source damage of 1,363, which is reduced by 363 damage. With 32.42% reduction it is 922, which is a reduction of 441. This means you took 78 less damage, which is 5.7%. It's the same reason 1% more reduction for a tank who has 70% damage reduction is so good. A hit of 21k on a tank is really a hit of 70k and 1% more reduction will reduce damage by 700, not 210. The relative gain can be expressed as: damage reduction gain / original-damage-taken. So with a 5.76% gain on top of 32.42% reduction you can note that as .0576 / .6758, or an 8.5% increase. The higher your damage reduction the higher the difference will seem.