Even though you are right by claiming that there is, for you non-warrior tanks, no difference between having 20%/10% to 15%/15%, you are forgetting the most important reason why you should try to equalise them.

Diminishing returns. Having such a high %'age of one above the other leads you to needing more rating of one to up your avoidance 1%. You need less total dodge+parry rating to get 15%/15% than you need for 20%/10%. This leads to you wasting statpoints just by gearing/regorging wrong.

However, there is a general consensus among deathknight tanks, it is reasonable to have one stat's rating 20% higher than the other. This would mean 12%/10% is acceptable.

Another note, if you ar using SwordShattering instead of Gargoyle, you should deduct 4% of your parry, so that would mean that instead of the normalized 12%/10%, you could use 16%/10% and still be "acceptable".

This is why when you're trying to equalise your parry and dodge you should look at the rating displayed on your character sheet instead of the actual percentages.

Garthar, warrior tank.
Skullknight, Unholy/Frost, with a tanking offspec.

Both on stormscale EU.