It's confusing but they answered it in the Errata:
The Actions section starting on page 190 should include a special
note concerning combining semi-auto and full-auto fire with the
Scatter quality, which reads “When firing a semi- or full-auto burst
at point blank range with a weapon that has the Scatter quality, the
extra hits for rate of fire and scatter are worked out separately and
both applied. For example, Horatius Kane fires his combat shotgun at
Heretic X. Kane is at point-blank range and fires a semiautomatic
burst. Kane rolls 01 with his modified Ballistic Skill of 70 (30 BS,
+30 for point-blank range, +10 for firing semi-auto) and hits by an amazing six degrees of success. He gets one hit at 70, one hit for
semi-automatic at 50, and a third hit for semi-auto at 30 (he does
not get a fourth hit at 10, because the combat shotgun’s rate of fire
is 3). He would get additional hits for scatter at 50, 30 and 10, for
a total of 6 hits on Heretic X, most likely shredding the cultist to
bits in the Emperor’s name.”
http://www.fantasyflightgames.com/ffg_content/dark-heresy/pdf/darkheresy-errata-v3.0-printable.pdf