How does the "LED offset settings" work ??

[AL1-1956]

Hello I have a 176 LEDS/meter connected to the RPI and the second LED is perfectly aligned with the first note of my 88 keys keyboard And every even LED is OK for the 70 first ones but then instead of the 72 one been lighted it's the 71 wich is lighted so there is now a mismatch. I tried to modify the LED offset settings , but as i do not fully understand how it works . So a little explanation will be greatly appreciated is the offset about the LED number or the Key number ? is it possible to input negative offsets ? I tried a few things but the results are quite odd (same LED for two different keys , LEDS not switching down when key released...) Thank You and have a great day

Alain