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.