A) the correct place to ask this is over at "Bob is the Oil Guy"
B) the specs for 5W-30 and 10W-30 are so close that Mobil (or any of the other) can manufacture a large batch of oil and measure it when it comes out of the process. The thicker batches goes in 10W-30 bottles, the thinner batches goes in 5W-30 bottles, and the spot on batches cuold be bottled in either 5W-30 or 10W-30 bottles.