A common question is "Where have my rankings gone" among the many who think that "rank checking" is a way of measuring how their optimisation and promotion is progressing and reporting to clients how well they are doing for them, well it is not the case any more.
A few years ago in the dim and distant past, we had the phenomena known at the "Google Dance" when there would be a single data push that synchronised all the datacentres at once, it lasted 3 days or so and the rankings were all over the place. When it ended, wherever your page was for the search phrase, was where it stayed for the next 3 1/2 weeks. Back then rank checking was an okay pastime because everybody saw the same thing.
No use if the phrase brought no traffic at all of course, but you could spend 3 1/2 weeks building up for the phrases that did work. So you did all the link begging, site tweaking etc etc and then held your breath for 3 days.
Then all of a sudden the "dance" ceased to exist, and the land was in turmoil. Actually what happened was that Google moved to a system of rolling updates, where each DC communicated with it's near neighbours and synchronised their copy of the index with each other.
The topology is probably a star pattern which minimises the propagation delays between each node, but I digress. The upshot of this is the datacentres are rarely if ever, synchronised fully, also because each DC has it's own crawlers and it's own copy of the ranking algo not every DC is in full possession of all the data about every page that is in the complete index. So the page you are looking for may not even be in that copy of the index.
Next point is that queries are handled by different datacentres on a rotating basis, so every query you make may come from a different DC, is the picture forming yet?
Then add into the mix the personalisation of search. Where, if you are logged in to a Google account, they will try to show results based on the search history of the client machine you are working from, and you have a situation where two people side by side in the same office can and do see different SERPs for the same queries. Then there is geo-location as well.
All in all not a good situation when you are telling your $1/2 million client that you are doing a wonderful job and have got them #1 rankings for lots of things. But, ermm, you'll have to trust me on that because you'll see something completely different if you actually go check.
Right. Ok then. Do you honestly think the next payment will arrive?