Your reading it right but your understanding of the efficiency is undeveloped. While the V18 produces 20W, like you see, it's made up of both PAR/visible light and heat. The efficiency is around 40%, meaning 40% is PAR - the stuff we and the plants are after; the heat, not so much. Take another look at the sheet upon page four. You will notice how the efficiency gets better at lower currents but less total wattage is produced.
According to Supra's 'cheat' sheets, 2x V18's (3000K, 3500K will be slightly more efficient) will bring around 23W (PAR) @ currents of 1050mA.
The V29's are more efficient, boosting possible PAR watts at given currents hence the prince difference.