How Does Acid Spores Actually Work?

The devourer, a Zerg unit in StarCraft: Brood War, has a special form of attack called "Acid Spores". When it attacks another unit, each hit adds a layer of Acid Spores on the target, up to 9 layers per each unit. This debuff causes the victim to receive 1 point of extra damage per layer and slows its attack rate. Word of dev says it increases cooldown of the said unit by 1/8 (12.5%) per hit, up to 9/8 (112.5%) of the original cooldown.

However, I suspect it's more complicated on the back end. Take the Corsair with a very short cooldown of 8. When it is fully coated in Acid Spores, it attacks slightly slower than the Mutalisk, which has a long cooldown of 30. This means that a full coating of Acid Spores (assuming that 9 spores is only 1 more coating over 8 spores) increases the Corsair's cooldown by at least (30/8 - 1) = 275%. What's going on here?

For comparison, I coated the Scout (which has a cooldown of 22) in 9 layers of acid spores and tested its attack speed. The affected Scout's cooldown is seemingly increased by about 110%, which is to be expected.

For now, I assume Acid Spores adds at least 2 in-game ticks to the cooldown duration of affected unit. This would explain why the Corsair receives, not 12.5%, but ~25% extra cooldown per each hit. More experiments needed, though.