Sram MRX Grip Shift not synching

[Tollyboll123]

I have a bike which i was given and I've found the Sram MRX grip shift will change gears on my front deraillier but the numbers on the shift don't correspond to the gear the bike is in.In other words when shift is on one the small cog is selected.When I select the middle cog the shift hasn't got to two yet.Its only on about one and a half.And when the big cog is selected the shift is only then on Two.But the bike runs fine.Does anyone know why this is and how to correct it so the gear and what the shifter says match up ??