They don't slash..they just moderate which is permitted under ICAI Regulations 39(2):
"The Council may, in its discretion, revise the marks obtained by all candidates or a section of candidates in any particular paper or papers or in the aggregate in such manner as may be considered necessary, for maintaining the standards of pass percentage provided in these Regulations.
Explanation — The term “section” used in this subregulation refers to the category of the candidates whose answer papers are valued by an examiner and such other category of candidates as may be specified by the Council."