This is a floating point error.
Any time you program math on a computer you need to account for these as the programmer. This includes even stuff like using Excel. As an example, start a new Excel file and enter the following values in cells A1:A3:
10.1
-10
-0.1
Then in cell A4 type the formula =SUM(A1:A3). The result will likely be similar to this:
Floating point occurs as computers can only us the numbers 1 and 0 to do anything, including math. As such values like 0.1 get rounded off to something near enough for any functional work (remember there are 16 zeros before that -3). There are a lot of resources on the topic of floating point math that you can use to learn more, but the ‘quick’ answer as as the graph author you need to account for this, usually by rounding to something like 6 decimals (or whatever accuracy suits the task).
PS: The video in this thread is likely worth a quick watch for more entertaining view on the topic: Appearing Decimals while filtering the Line Curve Lengths How To Avoid This error? - #2 by jacob.small