You know, I've wondered about this, too. Especially since Bryer's neighbor's mom mentioned seeing Kam all the time at WM as an employee. Would you really remember an employee who's only been there about a month that well, even if he was helpful, which she said he was, compared to one who's been there for several months?
Bryer's dad is the source for them both working there for 5 weeks. I think it's true Bryer was there for only 5 weeks, but it is certainly possible Kam was there longer, especially since he graduated the year before, and that Bryer's dad just wasn't aware of this fact. It would be less weird for Kam to have been working while Bryer was finishing school rather than spending a whole year apparently doing nothing, and it certainly may have been easier for them to get the same shifts if Kam had been there awhile and could pull strings. Might also explain where some of the money that Bryer's great-uncle referenced they left with came from.
Actually, some news articles mention them working there for several months, so maybe they did start at different times, and people are just assuming their time was the same, whether it was months or weeks. Hmm.
Edited to add source for neighbor's mom's statement:
RCMP continue search for suspects in three B.C. slayings