There's a couple of ways of looking at it, the way I see it.

Suppose ward parry % is half your normal parry % (no idea if this is right).

Player 1 parries 70% of attacks, with ward that's .70 + .30*(.70/2) = 80.5%

Player 2 parries 50% of attacks, with ward that's .50 + .50*(.50/2) = 62.5%

So player 1 is parrying 10.5% more attacks, and player 2 parries 12.5% more attacks. Seems like a win for the low-parry rate player, right?

However, think of it as reducing the damage you take from pre-ward to post-ward.
Here, player 1 now takes damage 19.5% of the time, compared to 30% previously, a 35% reduction.
Player 1 now takes damage 38.5% of the time, compared to 50% previously, only a 23% reduction.

In the end, you need a combination of these two approaches, since true total damage taken is a combination of parryable and non-parryable damage.

To keep making numbers up, suppose 2/3 of incoming damage is parryable (absorbing dodge and block into this) So starting from a baseline of 300 damage:

Without ward, player 1 takes 100 + 200*.3 = 160 damage and player 2 takes 200 damage.

With ward, player 1 takes 139 damage, a 13% reduction. Player 2 takes 175 damage, a 12.5% reduction. Pretty similar, right?

Basically, the more damage is unparryable, the better off player 2 is, since a strict reduction in number of hits is more meaningful, percentage wise.