Auto Numbering


On my automatic numbering, it went from DCR036 to DCR051 and I have no Idea why. Well I tried to reset it but every time I redo it, delete column, save, add new column, save start from DCR001, keeps increasing instead of starting from 1 again. How do I have it to start from DCR001 again, starting from Row 1 and so on? Thank you!!