working time Bookbinder, Barrack and Provision House

I saw something odd... i recalculated it a lot of time, but cant figure it out.

This problem effects Bookbinder, Barrack and Provision House, but for explaining i take the Bookbinder.

When a bookbinder is lvl 3, it takes 28 hours to make a codex.
When a bookbinder is lvl 4, it takes 21 hours to make a codex.

But when you'r going to upgrade the bookbinder from lvl 3 to lvl 4, it say: increase production speed by 33%.

But when i calculate it, 33% faster then 28 hours, should be less than 19 hours, not 21 hours. Am i a noob in calculating of is this really wrong?
This is all the same with upgrading to other lvls.

Who can explain this?

Quote:

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Originally Posted by Jamdoggy

The calculation is actually (time taken at Lv 1) / (Barracks Level)

So, if it takes 60 minutes to make 20 recruits at Lv 1,
Lv 2 = 30 mins
Lv 3 = 20 mins
Lv 4 = 15 mins
Lv 5 = 12 mins

So, as Robert1901 said, the percentage is the increase in production in the same time period.

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Found this here that helps explain the issue.

Again, this is someone getting mixed up over speed & time, one being a reciprocal of the other. The build time is not 33% less, the build speed is 33% more.