It is my understanding that all 403(B) contributions, whether elective or nonelective, reduce the elective deferral limit for qualified 457 plans.
I've tried to figure out why this is so, and have concluded that the purpose of 457 plans is to allow employees an opportunity to save money for their retirement when their employer may not be inclined to provide a nice retirement plan due to budget constraints, etc. that state & local governments face all the time. In those instances where employers DO contribute to a retirement plan, it theoretically reduces, on a dollar for dollar basis, the need for the employee to defer their current income.
My theory may not be correct, but at least it makes a believable story! Hope this is of some benefit to you.