Are employers allowed to offer different benefits to different employees?

Yes, there are not only allowed to offer different benefits, but most corporate group insurance policies often already have inbuilt variable covers.
Let us take the differences that are possible in group health insurance policies offered by any company.

1. Sum Insured – Most organizations offer a higher sum insured to senior-level employees. 
2. Room Rent – In those cases where room rent is provided as a percentage of the sum insured, those at a senior level who have a higher sum insured will automatically get a higher amount of room rent applicable to them than others. For example, there are two categories of the sum insured in a company – Rs 3 lacs and Rs 5 lacs. The room rent allowed is 1% of the sum insured per day. Those at Rs 3 lacs sum insured will get Rs 3,000 per day eligibility compared to those at Rs 5 lacs sum insured who will get Rs 5,000 per day. In this example, it will cause apparent differential eligibility. Sometimes in organizations, the top management may get a single AC room legibility or no room rent capping. 
3. OPD Benefits – in a few organizations, we have seen an additional OPD benefit being available to the senior level employees as well
4. Other Benefits – these include lower capping in the policy like treatment capping and higher maternity capping 

Any insurance benefits given in companies, including group health insurance, group personal accident insurance, and group term life insurance, are not a statutory benefit. This means that it is not mandated by the law of the land to provide such benefits mainly. Therefore it is the companies decision to offer group insurance benefits in a manner they consider appropriate for them. 
One way to view this is that many other benefits like travel allowances, car allowances, entertainment allowances also differ for the senior management employees. In the same manner, insurance benefits may also vary.
Can you contest differential benefits insurance given to the employees? Unfortunately, you can not since the organization voluntarily offers this group insurance to its employees. You can always give your feedback to the management or HR in your company to give the same or similar benefits across all levels and hierarchies.
Are there companies that offer the same insurance benefits to all their employees? Yes, many organizations provide the same benefits to all their employees. Such organizations are very few and far between. One of the reasons why most organizations do not follow this practice is that in such a scenario where they give the same benefits across all levels, they increase the cost of insurance as a proportion of salary for junior employees. Organizations often avoid this change in compensation structure.