Currently, public sub counts are abbreviated in most but not all places across YouTube. In August, we’ll make this more consistent by always showing abbreviated sub counts publicly. Creators: You’ll still see your full sub count in Studio! Learn more → http://yt.be/help/abbrev-count …pic.twitter.com/pmaKk4EU4k
The only explanation I can think of for this is that it’ll discourage subscriber count races, slightly increasing the relevance of sub count to engagement, but I don’t see the point since it’s already irrelevant in so many cases.