Demo Stat Choice

[Khadjid]

right. back to 18 seconds never will it go over 18 seconds

Only if you clip it, then it's just an illusion.

Corruption will have a longer duration at some points of haste. The point at which you get an extra tick is rounded up.

For example, to get your 8th tick on corruption, the calculations are as follows:
Tick frequency needed: 18s/7.5 = 2.4s
Haste needed: 3s/2.4s = 1.25
Haste rating needed (assuming DI + 5% haste buff): ((1.25/1.05/1.03) - 1)*12805.7 ~= 1995 (which is the haste plateau that we all know of)

With 8 ticks on corruption ticking every 2.4s, the total duration is 8*2.4 = 19.2

As you can see, dots can have a duration longer than its base duration, even without the extra tick from clipping them. Your assumption that they can't would require dots to gain the extra tick exactly when you can fit another tick in the duration, and not being rounded as it actually does.