I think it has to do with YVR's customs.

Of all the North American gateways I know, YVR seems to be the only place I have been asked to go through that 'questioning' session several times. Not in YUL, YYC, YYZ (not to mention any U.S. gateways, maybe except for Portland, where DL pulled out its Japan flights apparently because of some problems many passengers had with the customs there).

I like YVR except this customs clearance thing.

I would also recommend that you tell them just the truth. I think you would also get by if you arrive late (when that baggage check section is closed). But not sure, though.