The y centre of the text mesh won't correspond to the centre of the bounding box. For example it uses line height to calculate the center - this means that the center won't consider the vertical variation outside the line, e.g. the hanging part of the letter g, etc. the x axis uses the cursor position, i.e. the position the next char will appear at as the measure of width, which most often is larger than the actual character / mesh itself. Eg. a centred line of text should appear at the same position if it contains the string AAAAAA, or yyyyyyy and certainly if it contains AAyyAAyy.
If you want the mesh centre, you should just get that from renderer.bounds and adjust the position accordingly.