I know EA doesn't do it because of the fans, I am glad they don't do it because of the fans. I think EA does it more because of investors and how they were sued over press releases about Battlefield 4. Even if BioWare boasted I think people would look for ways to poke holes in what is being said, I remember many different posts ripping apart the investor press release where they said something about the initial sales being the best for any BioWare game and that was twisted a lot.
That's because in these statements, you make yourself appear as good as possible, so you tell the good things in the best way possible, carefully phrased as to not be caught in a lie later, and omit any problems.
In this case, we can be pretty sure that DAI had the best sales at launch as they said, though we don't know how long a time period is considered part of the launch, but we can also be pretty sure that DAI didn't continue to be Bioware's best-selling game, because were that the case, they would've said that. Nothing in all that is bad - because "DAI did not continue to be Bioware's best-selling game" isn't saying anything bad. It's just not the best that could be said. Still, it is always advisable to pay attention to the things not said.
As for not bragging more, there is probably an order from high-up to say nothing about sales, which is normal in a publicly-listed company. Anything they're allowed to say would be another statement that doesn't say much.