I just did some exhaustive research on this issue, and think I might be on to something.
Every time you do a search with the Google Keyword Tool you get two result sets back.
1 - More specific keywords - these are variations of your original keyword. These results are helpful because they allow you to find new variations and possible negative matches.
2 - Additional keywords to consider - these are synonyms and related keywords that can help you expand your keyword list.
Try some of these searches using the Google Keyword Tool - [url]https://adwords.google.com/select/KeywordToolExternal:[/url]
For non-branded terms, like the examples above, you get a healthy serving of "more specific keywords" and "additional keywords to consider"
Now try this same test with some branded terms. For this exercise it will be helpful to use known trademarks - e.g. iTunes
Note that most of the trademarked terms return few variations (if any), while the non-trademark terms return several variations. One could easily argue that the non-trademarked terms have more variations - thus more results. However, I did some keyword research on some of these trademarks earlier in the week and found hundreds of variations. Today I'm not seeing them.
I have been able to find several exceptions (e.g. canon camcorder), but I've found dozens of trademarks are no longer returning keyword variations - just related terms with no brand names.
Do you think Google might be trying to prevent search marketers from discovering variations of trademarked keywords?