Saying that OD7 DT isn't OD9 is pointless.
pp takes that OD7 DT = OD9, which is true for the hit windows of 300s, but if it took OD7 DT's actual hit windows for 100s and 50s, non-SS scores would give more pp compared to the same OD9 scores, because they are actually narrower than nomod OD9:
OD9 is 24ms/66ms/108ms, OD7 DT is 24ms/54ms/85ms, which makes it harder to get 100s in OD7 DT than OD10 (58ms), and harder to hit the notes at all compared to an hypothetical nomod OD11 (88ms).
What makes OD7 DT "easier" is that AR8 DT maps usually have easier patterns with less overlaps and easier rhythms than nomod OD9 maps, which have more complex patterns, rhythms, and have lower AR to begin with.
Also, nomod OD9's wider hit windows means that the notes' hit windows overlap more, which makes it harder to recover from misreads and misaims.