Vacuum reading varies depending on P or D?

[jh]

I just installed an Autometer vacuum/boost gauge in my '98 V70 T5.

When idling with the brakes applied:

- In Park or Neutral the reading is 20 in Hg
- In D the reading is 16 in Hg

Is this normal? In both cases, the readings are steady (they don't start higher and settle to that value). The rpm is the same when in P, N or D (one tick below 1000).