In a nutshell, this is how its calculated. Assume your car is for 100K,

1st year, total insured value is 100k,
2nd year, insured value is 80K (100-20 depreciation)
3rd year, insured value is 60K (100-40K depreciation)
4th Year, insured value is 40K .. and there on.

your car being in the 3rd, year, the insurance company has charged a 40% depreciation, that is what they meant by your having to pay 40% of the spare parts.