True, the number does not represent sales 1:1 because
1) There is a fail rate, but at the same time, it is only ~0.7%.
2) What gun store owners I've spoken to have told me is that in the last few years the number of multiple purchases per single check has risen dramatically (I've done it myself).
This means that now for every 1M NICS checks, if we subtract a liberal 1% for fails, then add 1 in 10 sales as a multiple, there are probably actually about 1.1M sales. So the actual figures of legal sales the last 8 years are very likely many millions higher.
And yes, the increase started in the Bush administration, but you can also see the average annual rate of increase in gun sales increased by 7.2x during the Obama administration as compared to the Bush administration.