Explain ONE reason why the male population aged 65 and over can be significantly smaller than the female population over age 65

One important reason is that most of dangerous job are performed by men, which means that the vast majority of people who died before age 65 because of job-related causes, are men.

Jobs like mining, truck driving, warfare even (soldier) are all dominated by men to a very large percentage (over 80 or 90%), and job accidents are not infrequent in these occupations, leading sometimes, unfortunately, to the deaths of the workers.