Inside information about CBSE checking.
Lemme give you a Lil Intel on the cbse checking too (I got my 12th results too today)
The teachers that are sent for evaluation aren't necessarily of that subject example for english checking maths teachers are sent and then asked to use the answer key to check the students paper. This lack of subject knowledge causes unfair marks deduction. The schools send random subjects teacher saying "we don't have enough teachers of that subject" and since there is no alternative thus, they use whoever comes. (This was followed in 10th checking especially)
The teachers who use OSM aren't very tech savvy which means that if a complex paper such as that of maths or science is sent to them, the probability of them assigning proper marks for each step and understanding where to do what on the PC causes them to rather just see the question and just assign average marks to that question rather than what was deserved.
This is the reason why students are scoring such low marks.
(Information from credible sources)