Bore Area = 100cm2
Stroke = Crank Throw x 2 = 10cm x 2 = 20cm
Therefore, Swept Volume = 100 x 20 = 2000cm3
4 cyls, 9600cm3 total
Cyl volume = 9600/4 = 2400cm3
Clearance Vol = Cyl Vol - Swept Vol = 2400 - 2000 = 400cm3
Comp Ratio = Cyl Vol/Clearance Vol = 2400/400 = 6
Therefore 6:1
I suspect you're taking the crank throw as the stroke, rather than doubling it.