I don't know. At every company I've worked for (as a junior, and as a senior) there were some juniors who clearly contributed more to the company's success than some seniors. I don't think this is an uncommon experience.
To me the "rank" of senior engineer should indicate the engineer's experience, and maybe their "wisdom" and depth of knowledge, not their salary. A senior engineer may slack off a bit and be less productive than an eager junior, or want to take on less responsibility; it seems fine in that case that the junior is rewarded with higher pay.
Know your value to the company, and negotiate compensation to match it (or exceed it!). I'd also suggest companies not tie pay to title.
To me the "rank" of senior engineer should indicate the engineer's experience, and maybe their "wisdom" and depth of knowledge, not their salary. A senior engineer may slack off a bit and be less productive than an eager junior, or want to take on less responsibility; it seems fine in that case that the junior is rewarded with higher pay.
Know your value to the company, and negotiate compensation to match it (or exceed it!). I'd also suggest companies not tie pay to title.