I can't imagine that being an accidental change. It's +1 to the base stat, not to the modifier, of course.
Note that the starting stat array has changed (16, 15, 13, 12, 9, 8 instead of 17, 15, 13, 11, 9, 8), and the stat score to modifier conversion table is also changed (12 is now a +0 rather than a +1).
Edit: I have run the maths. The biggest boost to statistic modifiers you can get out of the old system is +(+3). The biggest boost you can get in the new system is +(+5). However, you can't quite obtain three modifiers at +3.