It is straight (relative to the ground,) because it is -17º. This is pretty simple geometry: the stem is -17º, the head tube is 73º, and 73+17=90º-- parallel to the ground. The added 7mm of reach is because of the respective angles of the two stems. If both stems being compared were 120mm, the -17º would still be an additional 9mm away due to the angle.

edit to add for the most recent post by the OP:

110mm 0º Compared to:
110mm +17º: 14mm shorter, 29mm higher
110mm +7º: 5mm shorter, 13mm higher

This is all based on 30mm of spacer height. The +7º stem with 16mm of spacer height is essentially the same as the 0º with 30mm (within 1mm of height and reach)