[Harp-L] reed swap mystery

["mox" <mox.g@xxxxxxx>]

Intrigued by the various possibilities of spiral tunings i.e.

D Dorian
blow :  d  f  a  c  e  g  b  d  f  a
draw : e  g  b  d  f  a  c  e  g  b

D Mixolydian (circular)
blow : d  f#  a  c  e  g  b  d  f  a
draw : e  g  b  d  f#  a  c  e  g  b

D Major (Ionian)
blow : d  f#  a  c#  e  g   b  d   f  a
draw : e  g   b  d  f#  a  c#  e  g  b

All 3 scales can be built from a standard Richter in C.

I found that retuning in the lower holes to be "fairly" easy
but when it came to those itty-bitty short reeds the going started to get rough.

I then realised that by just swapping the blows & the draws 
from holes 7 thru 10 would avoid a whole lot of filling.

Now for the mystery mentioned in the title ;
The resulting pitches on the blow plate are all a semi-tone flat!

Can anyone explain what's happening?

Thanks in advance
Mox