Why does "numbers of people" take "has" not "have"?
Expenditure on direct payments has risen as numbers of people using them has increased and now equates to £1.1 billion or 6% of overall gross expenditure for adults.
Source: An overview of the UK domiciliary care sector February 2013 PDF file
Our freedoms are being forfeited as our numbers grow. Quality of life is reduced as numbers of humans increase.
Source: San Francisco Chronicle, EDITORIAL; Pg. 6; Letters to the editor; via COCA
Grammar Glitch Central's take on “Numbers of participants has…?
Edit: more examples
Two Medway Grammar Schools, Chatham Boys and Chatham girls have over a hundred spaces between them, as numbers of children in Medway drops sharply.
In both cases, divergence from the 7:3 ratio will be deemed to be drift, in retrospect at least, if the divergence declines as numbers of tosses or generations increases.
Source: Darwinian Reductionism: Or, How to Stop Worrying and Love Molecular Biology, Alexander Rosenberg, 2008 via Google Books