You have SAP_BASIS version level lower than 7.40
"Security levels" fields have values:
ALL = Critical - Red
Critical = Important - Yelow
Severe and Critical = Non critical - Green
After you will have SAP_BASIS 7.40 security levels will be less confusing:
High = Critical - Red
Medium = Severe - Yelow
Low = Non critical - Green
I hope it is clear for you now.