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
Psychology 101: The more people shown having an interest in something (like a youtube channel) will tempt others into joining. People like seeing "I can be the 1,000,000th subscriber!!!" with a 999,999 sub count. It makes them feel special. This ruins channel growth a lot.