A plausible explanation:
In April 2006, about one month before the Census, an anonymous e-mail written in French was circulating on the Internet. It contained false information and urged bilingual Francophones not to report, at the Census of Population, on May 16, 2006, that they knew both official languages, purportedly to ensure that the federal government would not cut services to Francophones. Francophones were encouraged to say that they knew French but not English. The e-mail spread across the country.
A note was posted on Statistics Canada's website alerting Canadians to the e-mail's erroneous contents and asking them to answer the questions accurately. Statistics Canada representatives also made a number of statements to the media in an effort to remedy the situation. In view of the data, however, it seems plausible that the e-mail influenced some Francophones in their responses to the question on knowledge of official languages. In the 2006 Census of Population results for this variable, the figures are probably low for the 'English and French' category and high for the 'French only' category, particularly for Francophones but also for the population in general. Users should therefore exercise caution in interpreting those data.