It has to do with a difference in reference level. For PCM audio—most digital audio like CDs, DVDs of all stripes, Blu-rays, FLAC, MP3, etc.—program amplitude level is measured against a maximum limit of 0dBFS ("zero decibels full scale"). Anything below that is expressed as a negative value below full scale. Violating full scale (i.e. positive values) results in distortion known as "clipping."
For DSD audio as found on SACD, Sony decided to set the 0dBFS reference level at the equivalent to -6dBFS in PCM. Because of this, it is possible to exceed "full scale" in DSD (not truly full scale) by nearly 6dB without clipping. Therefore, when converting DSD to PCM, you may find that the resulting audio peaks well below what you might expect from native PCM. I don't, however, recommend applying a blanket +6dB of gain to all PCM conversions, as many SACDs peak above 0dBFS, or -6dBFS In PCM terms. Instead, I would convert to PCM and then normalize to a sensible level on a per-album basis (to preserve inter-track dynamic relationships). I find -1dBFS to be a fairly safe normalization target.