Its easy. You get enough seniority to where you can hold a line and then you realize you can hold the next highest paying seat, but you'll be on reserve, so you bid it hoping to get a line soon. Then when you can hold a line, you immediately bid up to the next highest paying seat, even though you will be on reserve again.

Its sort of like that meme going around Facebook that says "I want six-pack abs, but I also want tacos."