I'm going to make an actual answer, to collect some things that others said in the comments, and add one my own, as I think it might be useful for future readers:
The actual text, as mentioned, is chosen from list of canned responses, so you can't read too much into it.
There is definite prejudice against minor edits to old questions. This comes from people clicking on a question they see in the Top Questions list, spending the time to read it, only to discover that it's mostly dead and done, but someone corrected the spelling of a word. In this case, as you mentioned, you were correcting the actual math, and that is certainly worth doing.
Lastly, and maybe surprisingly, I think you should have been more aggressive in your edit. See, the site culture here treats answers more like Wikipedia articles than Reddit posts, in that we are perfecting (well, at least improving) the site's content, and are less concerned about maintaining a record of the conversation that takes place around that. So just go in there and fix the formula - you don't need to add the correction as a footnote, along with an "Edit:" notation. My guess is that if you had done so, the edit would have been accepted.
And thanks for spending the effort to fix the broken things that you notice here!