Check the equation used for calculating chlorophyll a, b and total. Maybe the equation is incorrect. May contain an incorrect constant number (factor). Because the quantity of chlorophyll b is less than the quantity of chlorophyll a, but it is a positive value, not a negative. The sum of ch a and ch b is the total. Thank you and good luck.
It could be due to incorrect abs reading by spectrophotometer, it happens sometimes. If not, above described reasons by Mr Majid could be one of the factor.