Theories turning upside-down

Normally we would expect an increase in consumer sales to shift AD to the right and so be inflationary, all other things remaining equal. However, yesterday’s forecasts suggested that significant sales last month would be one of the main drivers of a reduction in inflation when figures for CPI and RPI were released – but the reason is the heavy discounting that retailers had to offer in order to gain those increased sales, so that prices actually fell even though the volume of sales rose.

Similarly, textbooks and historic data will show you that RPI has been higher than CPI, but that position is now reversed - while the new figures showed that CPI has fallen heavily to an annual rate of 3.1%, from 4.1% in November, the headline RPI rate of inflation dropped below 1% to 0.9%, from November’s annual rate of 3%. Note that CPI is still above the target of 2% plus or minus 1%, and yet tactics to boost the money supply, which is usually associated with increasing inflation, are at the centre of the government’s plans to encourage lending.
This is in spite of the rise in food prices during 2008, which is driving consumers to change their behaviour to shop around more in spite of the shoe-leather costs.
AS questions to consider:
Why is RPI lower than CPI at present?
Apart from discounting, what other reasons have caused inflation to fall?
Why is deflation a worry, and not a good thing?