But what does it mean for a +3 modifier to be assumed?
I just rolled up a character and got 16, 12, 18, 17, 14, 18.
Does this break the game? If not why is it possible? And if it's not a big problem then why is a 14 in a stat instead of a 16 somehow outside the maths of the game? Why are we assuming the game is designed for a specific number rather than say anything from +2 to +5. Surely, it would be better practice to design with a range in mind rather than a specific number. In any case is +3 is assumed it should probably be assumed only to the extent that something has to be the middle point of the range. It seems to me they key point here is not that your on that number, but that you're not too far from it.
Also if you're rolling dice in order you may end up with a Dwarf with 13 Strength and 20 Con. This is a perfectly viable Fighter (probably a very good Barbarian - you'll have good HP, good AC and you can afford to Reckless attack an awful lot, which means you won't miss often.).
I think people focus too much on what's easily visibly and quantifiable.