Part of the old pay formula was range - and the 319 has a higher range than the 320. Also, it was just pure negotiation. The 320 rate is a composite rate based on an approximate mix of 319/320 flying one might do and former NWA brought that rate to the table. Delta obviously chose to negotiate a higher rate for the 90 when it came on the property. No mystery - just different negotiating tactics by the respective groups.

Since the rates have now been established, you will unlikely see anything other than straight percentage raises in the future since most people would be unwilling to expend any negotiating capital to bring one aircraft group up over another.