Why this period of working overseas is not exempt for principal private residenc

Hi Sir, I have come across this question from the BPP revision kit. The following except is from the question concerning the occupation and deemed occupation of principal private residence for exempt chargeable purpose: The total ownership of the house is 210 months. The details of the occupation are as following: 34 - Occupied 18 - Unoccupied - Travelling overseas 24 - Unoccupied - Required to work overseas by employer 11 - Occupied 30 - Unoccupied - Required to work elsewhere in UK by employer 22 - Unoccupied - Travelling overseas 26 - Unoccupied - Required to work elsewhere in UK by employer 17 - Occupied 12 - Unoccupied - Required to work overseas by employer 13 - Unoccupied - Travelling overseas 3 - Unoccupied - Living with sister I understand the last 18 months are always regarded as deemed occupation; but why the balance of the 12 months unoccupation that was required to work overseas by employer (the third from the bottom) is regarded as chargeable months: 12 - (18-3-13) = 10? I thought that any time employed overseas are deemed occupation. So why that 10 months are not exempt for chargeable months? Thanks.