Are they calculating a share of overtime payments or an increase in payments due to an increase in your disposable income ? both are standard whilst you are in an IVA
You should have an expectation that the amount you pay will increase as you earn or or as your expenses decrease.
If you own your own property make sure you understand now what happens at month 54.
IVA started March 2011, Completed March 2016 and certificate issued 11 days after final payment. It was not always easy but then some of the best decisions aren't.