Every news source has a biased spin these days, yes some WAAAY more than others. But taking away the spin, are the actual numbers incorrect? To the armchair captain it looks like they were off by an amount equal to 40%+ of this deficit number in just 1 year's time. What if they continue to be this optimistic? Think of all the people quoting 2015 massively incorrect projections for an entire year, all the while snearing at those who dared to read news sources the aca proponents don't like.
I wouldn't sneer at anyone who came to a different conclusion than the CBO so long as they approached the process in an objective and statistically rigorous manner. The thing is that all of those people 'daring to read news sources the ACA proponents don't like' never, ever do this. The CBO is not always right, but they are right-er and more often than people who are coming up with convenient theories based on their personal political opinions.
TL;DR: CBO isn't perfect but they are vastly more accurate than listening to partisan news sources. People who do that should be sneered at.
Tl; dr
'Still pretty good' doesn't mean we should ignore missing not only the bullseye, but the whole dart board.
This hardly qualifies as anything close to missing the dart board.