The BIC approach collects less meth where meth is present. Go over twice not three times and spread the methanol further. So pick up less.
Still points to meth occurring over 6 months not 10 days. Meth habits do not go away. If the level of 2.4 was built up in 10 days, then after a further 3 months of occupancy by the same tenant, you would expect to see a much higher level than 4.3.