Jealousy is quite common. You see plenty of instances of this, people can't wait to measure their self-worth relative to others and would happily affect the lives of others in a negative way simply because it makes them feel better about themselves. After all if you determine your status by the pay you receive and the stuff you can buy then it's better for you if your colleagues earn less than you and your self-worth will suffer if your colleagues earn the same.
I had this at one job, I was a developer and earnt considerably more (though market rate) than the office manager.
She had a vendetta against me from day one with constant jibes about pay and such as prior to me starting she'd earnt the most out of the staff there.
It progressed into her making a landgrab (decided that I had to clear all days off and holidays through her etc) which I countered by speaking to my boss (and the company owner) who then told her that I was nothing to do with her and she should get back to managing the job of an office manager.
If I were not so thick skinned it would have been a deeply unpleasant experience.
Since I can't edit and forgot to say, should my company ever grow to the size where an office manager role is needed it will not be called Office Manager since you give some people the word Manager anywhere in their job title and they start planning to invade Poland.
Not sure what I would call it, Office Support Specialist or something suitably ambiguous I guess, the role is largely one of keeping the lights on, making sure the ISP is paid etc it's not a management role in the sense of dealing with staff etc.
Human nature is predictable and depressing.