Dan, yeah, we're talking two different things... but we're comparing the two bags the OP asked about. Why would someone compare the two bags? Because their weight is within 2 ounces.
More importantly, the lighter bag is also the much warmer bag... which makes it an even more interesting comparison. The EN comfort rating on the Atom is 50*F, the lower limit is 43. Western, though not EN rated, is known for rating their bags at the comfort level. So a bag that could be 20*F warmer, but in which you can shift the insulation to create a 50*F bag, is two ounces lighter. That's awesome!!
Western doesn't often go on sale, but if you keep an eye out you can sometimes find store-wide 15-20% off sales. Your atom was just over 20% off w/o the coupon you had. If we're trying to make the comparisons as equivalent as possible, I think it's fair to compare MSRP.
This isn't all to rip on the Atom. It's a great bag in some ways! But if we are indeed comparing these two, it's clear that the SummerLite is the winner on specs.