I would double check your TA test and make sure that you don't have static electricity on the dropper tip that is squirting drops instead of having well-formed hanging drops. Wipe the dropper tip with a damp cloth first. There's no way that 1 pound 12 ounces of baking soda (about 2.8 cups) added to 12,000 gallons increased TA by 50 ppm. The PoolMath calculation is right. Something else is wrong at your end -- the TA test, the pool size, or the amount of baking soda actually added.
Also, if you tested too soon in the area you added it, then you could be measuring a locally high TA area and instead need to wait 30 minutes with good pool circulation before measuring.