Question: Conversion of Array to List

Hello people in mapleprimes,

I am writing this sentences, hoping to be given any answer from you, to the 
question I have, about conversion of Array to List.

restart;
a:=plot3d(theta*z,theta=0..2,z=0..1,coords=cylindrical,grid=[3,2]);
A:=op([1,1],a);

brings a response of Array as, which I call BB,

Array(1 .. 3, 1 .. 2, 1 .. 3, {(1, 2, 3) = HFloat(1.), (2, 2, 1) = HFloat(.540302305868139765), (2, 2, 2) = HFloat(.841470984807896505), (2, 2, 3) = HFloat(1.), (3, 2, 1) = HFloat(-.832293673094284814), (3, 2, 2) = HFloat(1.81859485365136342), (3, 2, 3) = HFloat(1.)}, datatype = float[8])

And, I converted it to List, with 

convert(A,list,nested);

The response of this is, which I call CC,

[[[0., 0., 0.], [0., 0., 1.]], [[0., 0., 0.], [.540302305868140, .841470984807897, 1.]], [[-0., 0., 0.], [-.832293673094285, 1.81859485365136, 1.]]]

, which is composed of three lists:
[[0., 0., 0.], [0., 0., 1.]]

[[0., 0., 0.], [.540302305868140, .841470984807897, 1.]]

[[-0., 0., 0.], [-.832293673094285, 1.81859485365136, 1.]]

Then, my question is following:

Seeing CC, I can see three segments are shown.
The first one is  the segment between [0., 0., 0.] and [0., 0., 1.].
But, I can't find any elements of Array BB corresponding to [0,0,0] above in CC.
And as for 0, 0 in [0, 0, 1] in CC above, the first 0 should be (1,2,1) and the second one should be (1,2,2).
But, there are no such (1,2,1) and (1,2,2) in BB. 
Despite of these, BB is converted to CC, from any reason I want to know and ask this question for.

As for [[0., 0., 0.], [.540302305868140, .841470984807897, 1.]] in BB, the circumstances are the same.
I don't know from where [0,0,0] appeared. Surely, as MESH requires three points, you might say that
such things might occur. But, it is only after I converted Array BB to List CC, with plot3d or MESH then having no relation 
to the present circumstances. So, I think that it is not possible to say that Maple added [0,0,0] or such things to Array BB

 as it is about plot 3D or about MESH.

So, my question is how the conversion of BB to CC could have occurred.

Thanks in advance.

May_16_2018_question.mw