unexpected zip compression of DNxHD file

AE CS5 on Win 7 x64

I have been provided two QT files, A1 and B1 each 1080p ProRes (not sure if they are 422 or 4444).
Each is about 1.5 minutes long and around 2GB size.
AE CS 5 recognises both A1 and B1 as: apch, trillions of colours.
If I zip these ProRes files I only get a very tiny reduction in file size (less than 1%) as expected.

As a test I rendered out both files from AE to DNxHD without doing anything to the videos.

Settings used:

AE project set to 16bit

QT container
Avid DNxHD Codec
trillions of colours
Q = 100%

Codec settings:
709 colour levels
No Alpha
1080p/25 DNxHD 185 10-bit

The resulting files A2 and B2 both maintain a 2GB file size and visually are identical to the originals, however if I zip each video file separately (with legacy settings) A2 manages 50% compression and B2 manages less than 1% as with B1.

* Both videos have a similar ratio of action to non-action (not that zip should be able to take advantage of this anyway).
* The amount of grain is comparable in both too, although A1 / A2 have a slightly softer look to them as if a very slight blur has been applied to the source video.

Could this softness be enough to account for the better compressibility of A2 and if so why would A2 compress to 50% when A1 did not?

Any and all ideas welcome!

Many thanks

Graham Macfarlane
3D animator and VFX specialist
London UK