Unless there's a wage bargaining agreement in place (e.g. through a trade union) for one ore more sector's of a company's workforce, every employee's contract with his/her employer is entirely independent of that of any other employee and the employer is free to pay everyone diffeently. The only exceptions are where there's a breach of equality legislation (e.g. if men were being paid more than women) or if part-time employees are treated differently to full-time ones.
In the City of London's financial businesses it's not unusual for people doing exactly the same job to get vastly different pay. For example one employee might be working for £40k (because he joined at the starting level and hasn't had any significant pay rises), whereas the guy sitting at the next desk and doing exactly the same job (but who was head-hunted by the company from a rival firm) might be getting £200k. It's all perfectly legal and long may it remain so!