WCAG-DI consistency of "perceivable unit" and "user agent" definitions?

The Device Independence Glossary [1] has definitions for the terms
"perceivable unit" and "user agent".  The wording of these definitions seems
to me somewhat different than the wording of definitions for "perceivable 
unit"
and "user agent" (same terms?) in the WCAG2.0 Glossary [2].

Definition of "perceivable unit", from [2]:
"The result of a user agent rendering the contents of a delivery unit. User 
agents may
or may not render all information in a delivery unit. In some cases, a 
single delivery unit
  may be rendered as multiple perceivable units. For example, a single html 
file that is
rendered as a set of presentation slides. Most perceivable units contain 
presentation
and the means for interaction. However, for some devices such as printers, 
a perceivable
  unit may only contain presentation."

Definition of "perceivable unit",  from [1]:
"A set of material which, when rendered by a user agent, may be
perceived by a user and with which interaction may be possible.
User agents may choose to render some or all of the material they
  receive in a delivery unit as a single perceivable unit or as multiple 
perceivable units.
Most perceivable units provide both presentation and the means for 
interaction. However,
  on some types of device, such as printers, perceivable units might 
contain only presentation."


Is there a reason for the definition of "perceivable unit" to be different 
as mentioned previously?


Definition of "user agent",  from [2]:
"Any software that retrieves and renders Web content for users. This may 
include Web browsers,
  media players, plug-ins, and other programs — including assistive 
technologies — that help
in retrieving and rendering Web content."

Definition of "user agent", from [1]:
"A client within a device that performs rendering.
Browsers are examples of user agents, as are web robots that automatically 
traverse the
  web collecting information."

Is there a reason for the definition of "user agent" to be different as 
mentioned previously?

  I notice that the definitions of the terms "authored unit" and "delivery 
unit" in [2] seem to have
been taken verbatim from [1].

Should there be consideration for taking the definitions of "perceivable 
unit" and "user agent" in [2]
verbatim from [1] as well?   If not, why not?

Thanks and best wishes
Tim Boland NIST

[1]: http://www.w3.org/TR/di-gloss/

[2]: http://www.w3.org/WAI/GL/WCAG20/WD-WCAG20-20060117/appendixA.html