Stat bonuses are a part of the base, where it goes from there is where the flatter math actually matters. Giving different PCs a higher stat bonus at the start makes them a bit more different mechanically.If the intent is to "flatten the math" and thus keep the numbers smaller for longer, why are the stat bonuses still linear (a la 3e) rather than bell-curved a la 1e?
If all the bonuses other than +0 had their actual number reduced by one (thus making 6-7 -1, 8-13 the +0 range, 14-15 +1, etc.) wouldn't that help keep the numbers smaller? Particularly if they went linear from there rather than accelerating like 1e did toward the extremes.