If you are making changes for previous payruns, the Year to date (YTD) value that you see will take into account the amounts for the other payruns done before.
For example, if you are editing and re-processing a January payrun in April (with Feb- Apr payruns done), the Jan payrun will reflect the YTD amount calculated from Jan to Apr (since Feb – Apr payruns are already processed). So if the value is 1000 per month, the figure will show 4000 (4 months total) instead of 1000 in January. This would not cause any errors in bank files and the YTD figure is only for references purposes.
However, if you wish to rectify this, you would need to delete all the affected payruns for the employee and rerun them in order once again.