That is the way it has been designed. Following is from the help for WEEKNUM;
""
The WEEKNUM function considers the week containing January 1 to be the first
week of the year. However, there is a European standard that defines the
first week as the one with the majority of days (four or more) falling in the
new year. This means that for years in which there are three days or less in
the first week of January, the WEEKNUM function returns week numbers that are
incorrect according to the European standard."

"WS" wrote:

Weeknum does not show 53 for the entire week of 12/28/08-1/3/09, it shows
week 53 for 12/28-12/31, and week 1 for 1/4-1/3/2008. The week number can't
be both 53 and 1