I don't know how it works for other student loan servicers, but I do know that when you go from a higher paying plan to an IBR plan, the lower payment is now shows up as your monthly payment. As it changes, so does the monthly payment amount reported. Also, I found that if you have a reduced income any month before your annual recalculation, submit your paperwork early. They will recalculate it even lower and that amount will be good for another year.
I alway encourage people to use the IBR plans (request the lowest plan on the application) because it has a loan forgiveness too.