Hi Karen --- it is down to timing really. You are just a little out of step. Your original income would have been based on a 10 month old figure too. The P60's just reflect one year to the next and really are to confirm that you didn't have any undeclared or overlooked additional pay the previous year.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014