General terms in Google Trends all seem to trend down

[koan]

It is a rather short and simple question: I have noticed that your average generic terms in Google Trends always seem to have a downward trend over the years, does anybody can explain this phenomenon to me?

I know we can't go into specifics, but just enter non growth industries related terms like "trees", "rocks" or "ocean", for example, and you can see it (more on some than others, yet it's always downward).

Is there a constant flow of new terms that leaves a smaller portion of the pie to general ones?

[tedster]

It might reflect the trend toward longer search phrases in past years.

We've only discussed Google Trends a little bit on WebmasterWorld to date, but this thread from last summer raised some interesting points:

Google Trends - how accurate is it? [webmasterworld.com]

[reprint]

I see this too across a very wide range of search terms. I tend to use Google trends differently in that I use a term as a benchmark and then compare other terms to that. Those that match or outperform are most interesting to me. Those that trend better though not exceeding the benchmark term are next in line.