Saying 'doubled' in Chinese 一倍 vs 两倍 I stumbled upon these 2 sentences:

这个城市人口比十年前增加了一倍

这个城市人口比十年前增加了两倍

The city’s population has doubled from a decade ago

I though 倍 always multiplies the number standing in front of it, it seems like in case of 一

How come? Is there any logical explanation?
Technically, when you say "增加了n倍", "n倍" is describing the difference the two number. So the original x becomes x*(n+1), while the difference is n*x.

If you want to say that the latter number is just n times the former, then it is "增加到了(原来的)n倍"

However, in casual speech, people may not always respect this rule, which makes it confusing. Sometimes we have to make a confirmation to make sure we get the real number.