Is it possible that Kent took leaves-of-absence for the birth of her two children? That would slow her down on the seniority track. Also, even with union contracts, pay hikes are sometimes possible for merit, whether within rank or at the time of promotion. (I'm not familiar with the ABT contract in particular, but know it's possible with other union contracts. I.e., it's a myth that raises are possible only through seniority for people with union contracts.)
I was intrigued at the McKenzie remark in the NY Times story that the guest artists are paid at weekly rates no higher than the regular principals. In the U.S., we sometimes forget that our compensation package includes not only salary, but also health insurance and a retirement program. Those are listed separately on the 990 forms. It's possible that foreign visitors with health insurance and retirement benefits from their home company have the option of taking the value of those benefits in cash salary. I don't know how ABT handles this, but it's certainly possible.
Mr. McKenzie said that all dancers are paid by the week, not per performance, and that the company did not pay its international stars . . . any more than its highest-paid principals. “I don’t, and won’t,” he said. “That’s just a policy.”