Mal:
I think the confusion stems from how destination URL trumping works in
a Google AdWords account.
In any Ad Group, you could have a collection of Ads and Keywords.
The relationship between Ads and Keywords is MANY-TO-MANY.
An Ad-KW combination is selected (by Google's algorithm and possibly
user hints) depending on the context of the search in question.
This could be weighted on the conversion results or using a simple
round robin algorithm.
For the selected Ad-KW combination, the following use cases arise...
i. Ad destination URL is defined; KW destination URL is not defined
ii. Ad destination URL is defined; KW destination URL is defined
Note: definition of an Ad destination URL (during creation of a Text
Ad variation) is mandatory/required and cannot be omitted.
For case (i), the KW destination URL will be used (it always trumps
the Ad destination URL)
For case (ii), the Ad destination URL will be used because the KW
destination URL doesn't exist.
You could determine the actual Ad and KW combination used at runtime
via a call to the DefinedReportJob API in v11 using...
http://www.google.com/apis/adwords/developer/DefinedReportJob.html
In addition, refer to dynamic keyword insertion abilities via
{keyword} or {creative} inserts.
This might help you in better tracking of URLs w.r.t. the specific
Keyword or Ad in question.
Hope that helps.
- Rohit