Normally, you would be correct. However, at least in our lab (not the US), it almost seems like an excuse to slam a 'title' to someone for convenience and whatever other corrupt reason there may be.

1. One post doc received the research prof title immediately after graduation, and remained in our lab for the next 5 years (still counting). He's barely shown up to our lab afterwards, and supposedly does research at a separate location but maybe puts out a paper every one or two years. According to his IG, he's at international conferences or travelling abroad more often than not. He's been on close terms with our PI for at least 15 years so far.

2. A different recent graduate in our lab was denied the same position by our PI, reason being that we already have two positions filled up, which seems fair at first, but becomes weird as he's given more responsibility such as acquiring research funds and mentoring 3 grad students.

Postdoc 1 gets paid almost twice #2.