They changed the % at the end of v1 so that all % are 3/4 the shown number.
IE
10% = 7.5%
50% = 37.5%
So it affects higher percentages a lot more than lower percentages but it affects all %'s.

Also re your earlier test 40 damage a hit every hit will add up fast, particularly with a weapon like twin sabers that has lots and lots of low damage hits. Cross Hurricane hits 25 times total over the course of the combo so thats a 1000 extra damage each combo. Its not a huge difference but it'll add up over the course of a run.
Grab an axe or a weapon with high atp if you can and you'll start seeing a much bigger difference in each individual hit.
(Also Twin DB's sabers have godly ATP for a-rank sabers, its one of the few weapons whose ATP will start offsetting other sabers attribute advantage - its the exception to the rule).