What happens with the [Value] variable inside two different GENERATESERIES

try removing the min order max order variables. Then the calculation of NextFaults should be nested inside generateseries where w is defined. 

Quality variation_COCHE2 =
AVERAGEX (
CROSSJOIN (
VALUES ( 'Tabla Proyecto'[PROYECTO] ),VALUES('Tabla Area'[AREA]) ),
VAR T =
CALCULATETABLE ( Hoja1 )
RETURN
AVERAGEX (
GENERATESERIES ( MIN(Hoja1[ORDEN COCHE PROYECTO]), MAX(Hoja1[ORDEN COCHE PROYECTO]) -1 , 1 ),
VAR Z = [Value]
VAR NextFaults =
AVERAGEX(
GENERATESERIES(Z, MAX(Hoja1[ORDEN COCHE PROYECTO]) - 1, 1),
VAR W = [Value]
RETURN SUMX ( FILTER ( T, Hoja1[ORDEN COCHE PROYECTO]= W + 1 ), Hoja1[FALTAS TOTALES (QA)])
)
VAR CurrentFaults = SUMX ( FILTER ( T, Hoja1[ORDEN COCHE PROYECTO]= Z), Hoja1[FALTAS TOTALES (QA)])
RETURN DIVIDE(NextFaults - CurrentFaults, CurrentFaults) * 100
)
)

see if it works. 

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