This could happen for WeekStarts=Sunday. The week number calculation is based on ISO 8601 which uses Monday as the first day of week. The supplied Sunday belongs to a previous week according to the standard.
It's fixed now in build 6.6.2336:
http://www.daypilot.org/daypilot-pro-6-6.html
It finds the Monday of the current week and passes it to Week.WeekNrISO8601() method.
The workaround would be to use custom HTML for the header using BeforeTimeHeaderRender event handler (for e.IsColGroup == true).