ehgee - 10:19 am on Nov 1, 2012 (gmt 0)

I meant information. What information is in my browsing history that would tell Google my next search is for something I want to buy?

Several things actually... for example, you visited a webpage of "carved widgets". You read it long enough and even added the item to the cart, but then abandoned it... BINGO, you've left a trace. All of this could be seen for yourself in analytics leave alone by Google. Just one way, among many others I'm sure.

I very rarely order anything online (if I find a supplier, I phone them: I don't want to buy anything from someone I can't gey hold of if there is a problem).

Sure, thats a percentage, Similarly, there's a percentage of online buyers... we're referring to this latter segment

Search phrases tell them more, but even there they can't always easily tell whether "widgets" means I want to find out about widgets or whether I want to buy one.

Again, refer my example above

It seems far more likely to me that Google are trying more and more radical methods to manage a mountain of content that is growing at an almost (and I am not even sure about almost) unimaginable rate.

Yes, but that does not explain stable rankings & traffic..