Let's not be short-sided.
OK, so the amount of fermentable sugar in the beer provides the potential for higher alcohol, but it's the alcohol tolerance of the yeast that determines the actual ABV. Weaker yeast cells die off from the alcohol level so ultimately the ABV is dependant on how tolerant the yeast are to alcohol.
Some yeast, like Turbo or Champagne and others can tolerate alcohol levels in the mid to high teens.
This is why people take gravity readings. At the beginning you can see potentially how high an ABV you could achieve. The final gravity reading shows how much sugar was converted prior to the yeast dying down to a level to where they can simply convert no more.
Also, the fermentability of the sugar plays a factor. Wort isn't just sugar and water, there's all sorts of matter that can't be digested by yeast. This is why wort can't get to the gravity of water or lower.
Honey can ferment to a level where the gravity is lower than water. Almost all of the honey is digested into alcohol, so the gravity can be (for example) .990 when water is 1.000.
Other times, brewers add non-fermentable sugars for "mouthfeel" (i.e. malto-dextrine) or to provide sweetness (i.e. Lactose).