I think it true, look at this:

06/13/2006: July 2006 Visa Bulletin

RETROGRESSION OF FAMILY F2A CUT-OFF DATE:For July, it has been necessary to retrogress the F1 and F2A cut-off dates. This has been done in an effort to hold the issuance levels within the applicable annual numerical limits for the affected categories.
IMMIGRANT VISA AVAILABILITY DURING THE REMAINDER OF FY-2006: Readers are reminded that during the summer months, with immigrant visa number use approaching the annual limits for the year, the supply of numbers remaining for use is limited. Thus, cut-off date advances could slow or stop, and monthly allocations decrease. Additional retrogression of cut-off dates such as those experienced for July are possible. Readers should not assume visa availability until the cut-off dates are announced.

By http://www.immigration-law.com/Canada.html