Money is always a factor. Certainly won't be the only one though.
Arbitrators always reject a simplistic comparison of last year's W-2s in creating the combined list. Here's what the Delta seniority ruling said about taking past or present circumstances and projecting them into the future:

On the one hand, dealing with the future prospects of anything in the airline
industry is nothing short of reading tea leaves or, to cite a far more daunting venture, predicting fuel prices. On the other hand, those sorts of assessments are the stuff of which “career expectations” are made. Therefore, it is appropriate that one examine possibilities and potentials to whatever extent is reasonable, in the course of constructing a merged seniority list that is fair and equitable.
...

An integrated list that responds solely to statistical absolutes (for example), with no broader view of the short and long-term impact on career expectations, might be considered nominally fair but realistically inequitable. Too, a process that ignores reality and bypasses facts, that pursues, instead, an illusory notion of “something for everyone,” could hardly be fair.
In constructing this list, we have inquired as to where the respective groups

have been and we have made reasoned judgments as to where they were going. We have attempted, at all times, to recognize reasonable expectations of both parties while, in all instances, rejecting proposals that, however facially logical, resulted in untenable windfalls.
The resulting list neither realizes nor maintains each and every career
expectations, nor could it do so. No recitation of career expectations ever includes a merger, and no merger can leave all hopes and plans unaffected.