No, GASherbet, it's not based on that at all.
It does in fact add 10%. And is internally consistent with every single other percentage modifier in the game
Percentages in civ games are additive, not multiplicative. What this means, is that your baqse value, and your bonuses, are tracked seperately. And to determine your final value (whether it be production, gold, combat strength, workrate, etc) the bonuses are first added together, and then that percentage is applied to the base value.
An example.
Say you have 100 base production in a city (you're running ROK and you have a LOT of mines)
In addition to these, you're khazad with overflowing vaults, which gives 40%
You have a forge there, which gives another 25% and an artisan's workshop, for another 20.
Under a multiplicative system (which we don't use) you would get:
100 *1.4 * 1.25 * 1.2 = 210 total
Under the Additive system, which is what every civ game has used as far as I can remember, you get:
40+25+20 = 85% bonus
100 * 1.85 = 185 total production.
As you can see, the bonuses are added together before applying to your base. What this means in essence, is that whenever you get a 10% bonus, it gives you an extra 10% of your base value, not of your current value
With workers, I believe all builds have a "cost"./ And workers add a certain amount to the progress on that improvement every turn, which is multiplied by their (workrate / 100) Thusly, taking skilled I increases thw worker's workrate by 10% of the base value. I'm not aware of any way at all to affect that base value