Well, the reason that is so because the ultimate limitation is 0.1% of the final product.
So, let's just compare some numbers...
Let's say we have oakmoss at 5% of the perfume concentrate. What percent is that of the final product? Well, that depends on the dilution level of the perfume concentrate. Let's do some math*:
EdC concentration - 3% perfume oil to 97% alcohol. Let's assume this is a 100ml bottle. 3% of 100ml is 3ml, so there is 3ml of juice, total. Now, 5% of that (the oakmoss) = 3 * .05 which = 0.15ml. That's the amount of actual pure oakmoss in the bottle. But what percentage is that of 100ml? Well, 100ml = 100% of the product so .15ml = 0.15%. That's still too high. With a dilution of 3% perfume oils, we could only use 3.33% oakmoss to end up at the 0.1% of final product IFRA limitation.
If we have an EdT at 10% dilution, we can only use 1% oakmoss in the perfume concentrate to = 0.1% of the final product.
If we have a EdP at 20% dilution, we can only use 0.5% oakmoss in the perfume concentrate to stay within the IFRA limits.
As you can see, the higher the ratio of perfume oil to alcohol (the higher the concentration), the lower the amount of oakmoss in the concentrate has to be.
*The easier math is to just multiply out 3% * 5% (.03 * .05) to arrive at the .15% number (.03 * .05 = 0.0015 = .15%) but as this is not as illustrative of the concept I went with the more long-winded version shown above.
I think what you are saying is...not that you will have MORE oakmoss in the bottle, because you can have 0.1% of oakmoss in any 100 ml, regardless of whether it is EDC, EDT, etc. However, you will have a larger PROPORTION of oakmoss in relation to the entire composition if you produce it as an EDC.