I doubt because you have 9 multiplications and you are adding 9 numbers together, when using a 3x3 convolution matrix.
Adding 9 numbers: 1+2+3+4+5+6+7+8+9
We are performing 8 additions as we repeat the operation of 'adding' 8 times (8 times a '+' sign).
We can say there are 9 additions as well, if you would accept a definition like: 0+1+2+3+4+5+6+7+8+9
But what does make more sense for computers: 8 adds or 9 adds?