My understanding of jets is that the hole in a 127 jet is 1.27mm. The hole in a 128 jet is 1.28mm. That's a diameter difference of 1/100 millimeter. That ain't nothin'. Don't stress over it. If you bought both sizes and tried them out, you probably wouldn't be able to tell the difference by the seat of your pants. (But a dyno probably could)
Pat's Jetting Calculator is a great tool to get you in the ballpark, but don't take it as absolute gospel. It's not based on extensive lab tests. It's based on what guys and gals say works good for them. Your results may vary.
Cody
