Larian Studios Forums Baldur's Gate III Baldur's Gate III - Suggestions & Feedback First Impression, One Hour In

I think it actually comes from PHB page 13 ; it's tied to the creating character variant which makes you buying points for ability scores :

"Variant: Customizing Ability Scores
At your Dungeon Master’s option, you can use this variant for determining your ability scores. The method described here allows you to build a character with a set of ability scores you choose individually.

You have 27 points to spend on your ability scores. The cost of each score is shown on the Ability Score Point Cost table. For example, a score of 14 costs 7 points. Using this method, 15 is the highest ability score you can end up with, before applying racial increases. You can’t have a score lower than 8. This method of determining ability scores enables you to create a set of three high numbers and three low ones (15, 15, 15, 8, 8, 8), a set of numbers that are above average and nearly equal (13, 13, 13, 12, 12, 12), or any set of numbers between those extremes."

So you shouldn't be over 16 overall given that 15 is the max prior to adding your racial bonuses (arguably you could get up to 17 with a +2 racial to one score).

Anyway, the game should be balanced around what ability scores you can get at level 1. As a DM I often boost characters when they roll for their stats so I can have an "epic" campaign thrown at my players.