From UNAIDS

In 2007, UNAIDS lowered the prevalence of the prevalence of HIV infection around the world. Why?

The reduction of 6.3 million is largely due to improved and expanded surveillance, data collection and methodologies, which have allowed UNAIDS and WHO to present a more detailed and accurate picture of the global AIDS epidemic.
Roughly 70% of the difference is explained by reductions in HIV prevalence in India (which alone accounts for approximately half the revision) and several sub-Saharan African countries, including Nigeria, Mozambique, Zimbabwe, Kenya and Angola. The remaining 30% of revisions mostly occurred in a number of sub-Saharan African countries. A decrease in incidence (the number of new HIV infections) may also be a factor, but is currently difficult to quantify.

Information from population-based surveys has been used to inform the adjustments in all countries. However, in some countries, notably Zimbabwe, Cote D'Ivoire and Kenya, the reduction in HIV prevalence is mostly due to actual declines, rather than statistical corrections.