I think that calculator ignores the Str bonus from Change Instruction since it used to disappear, so Str ratings on a Vanilmirth will look higher than they actually are. Any Vanilmirth will have 4 less Str than it calculates there if it has Lv. 5 Instruction Change (in this case, that means Str -6 rather than -2). D:
As far as I know from extensively looking into it, and unless our coding has been altered, yours was totally unlucky.
According to the code I saw once upon a time, it's possible (but dismissably unlikely--you would have to be the most unlucky statistic to ever exist in anything) to have absolutely no stat growth from level 1 to 99 aside from evolution. There have been many instances where one of Vanilmirth's stats--usually Int, since it's what's looked at most--would not grow at all for 17+ levels and all that. My previous Vanilmirth gave up on Int at 85+ even though it was riding the top of the "Amazing"/"Ridiculous" stat up until that point. '''O|¯|_
