The "proportionate decreases" comment refers to persons on payments who receive less than the full rate because of earnings limits.
For example, a person who applies for Illness benefit may have the appropriate number of PRSI contributions to qualify for payment, but because their earnings are less than a set limit, they are not entitled to the full rate of Illness Benefit.
It is these rates that are reduced proportionately - i.e. the €8 reduction is not applied in full.
I presume the reason you receive a reduced rate of Disability Allowance is because there are means assessed against you (e.g. savings, 2nd property, other income).
The means assessed is subtracted from the full rate of Disability Allowance to arrive at your weekly entitlement.
As the full rate of Disability Allowance has reduced by €8 (for single person) and €13.30 (for a couple), your weekly entitlement is reduced by that amount also because your means have not changed.
If you believe that your means have changed, you can ask for a review of your claim.