Can I rely on Google CSE results accuracy compared to google.com?

Question

I have been testing a CSEs accuracy in comparison to google and it seems to fall down when I type in full urls with long query strings. Shorter keyword based and nice url pages are coming through fine.

At first I just thought the pages were not indexed, but they are on google.com and google.co.uk, the only problem is with my CSE. Hence the confusion.

Does anyone know if there is a fundamental difference between:

1. The ranking algorithm used
2. The datasets being used
3. The datacenters being used.
4. Anything else.

I have tried only allowing the specific site, as well as allowing results from the entire web.

To put is basically, can I reliably expect a CSE and Google's results to match or be very similar, assuming no variables?

Answer 1

No, the mismatch between google.com results and CSE results is a known issue. Google has said that they value speed of results over completeness, and that's just how it is.

This answer has been the same since 2007:

http://www.google.com/support/customsearch/bin/answer.py?hl=en&answer=141877

Answer 2

I've noticed that in search results by CSE are missing the ones from forums.