Why stats drift in long serials
A stat that changes in chapter 7 has to be right in chapter 70. Over a long serial the numbers end up in your head, in old chapters and in a spreadsheet that quietly stops matching either. Logging each change against the chapter where it happens gives you one place to look: pick a chapter and you see what every stat was then.
For catching the mismatches that do slip through, read how to catch continuity errors in your stat blocks and how to keep stats consistent across a series.