Draw bounds are different from layout bounds. Below, the draw bounds are outlined in green. Layout bounds are outlined in blue.
The draw bounds are the area that is covered when you draw the text. Notice how the rectangles don't necessarily align along the top or bottom edge. And how, in some cases, the bounds may overlap each other.
The layout bounds align along the top and bottom edges, and are guaranteed never to overlap each other.
Both types of bounds are useful for different purposes. Layout bounds are useful for positioning text, aligned against an edge. Draw bounds are useful as a clip rect.