The message showing the number of characters that are still necessary to reach the minimum comment length, or the number of characters that you can still type is shown on browser side, while the validation of the comment is done server-side.
The error message could be changed as the following, but then it should list all the characters that are removed.
Comments must be at least 15 characters in length. Trailing spaces are removed.
For example, 15 spaces followed by a back-tick still cause the error to appear.
Probably the message should be similar to the following one, if it needs to really be changed.
Comments must be at least 15 characters in length. Some characters are not considered when calculating the comment lenght.