Some background - a study here, reported:
Although service academy graduates have historically attained flag rank at a much higher percentage rate than ROTC and OCS
graduates, the significant shift occurred between 1972 and 1990. In 1972, only about 20% of flag officers were from ROTC and OCS combined, whereas in 1990 over 50% of general/flag officers were from ROTC and OCS commissioning sources. Differences in
career progression between the three main commissioning sources were indistinguishable in the GAO Report.