You're forgetting the the falchion and the battleaxe have different base damage, and the battleaxe is 1H while the falchion is 2H.Tzarevitch said:Even in 3.5 a falchion can still get a whopping threat range of 15-20 (3 in 10) x2 with even 1 expansion of its threat range (keen or improved critical). The poor battleaxe however is now forever stuck at 19-20 (1 in 10) x3. The 3x greater frequency of crit hits will significantly outweigh the slightly greater damage multiplier on average. The +1 threat increase solution allows the narrow-crit weapons to keep some parity while still limiting threat range increases overall.
What you seem to be missing is that the martial weapons are all perfectly balanced with one another in terms of base damage, threat range, and critical multiplier. Weapons of the same size have different properties but pretty much do the same average damage. Battleaxes are distinct but basically the same as longswords, for example. In order to maintain that parity when the threat ranges are increased through feats or magic, you need to increase the range by its original amount, not just a flat +1. The +1 across-the-board increase gives a bigger boost to the small threat-range, large-multiplier weapons over the others.