The MultiCell is used for print text with multiple lines. It has the same atributes of Cell except for ln and link.
$pdf->MultiCell( 200, 40, $reportSubtitle, 1);
What multiCell does is to spread the given text into multiple cells, this means that the second parameter defines the height of each line (individual cell) and not the height of all cells (collectively).
MultiCell(float w, float h, string txt [, mixed border [, string align [, boolean fill]]])
You can read the full documentation here.