Additional Tax to 18% after Leaving before 183 Days?

I had been a resident of Taiwan for a number of years but recently left my job, and the country, mid-contract. (I was a teacher.) This year, 2010, therefore, I spent only 85 days as an employee. Now, outside of Taiwan, I am trying to claim at least part of my income for February–one week–and March–four weeks.

In a message, my employer sure enough claims that I owe 18% on my total income, since I left well short of 183 days. Under the new rules, painful but fine. But he also tacked on a 12% charge “from the previous month, when the tax was 6%.” This smaller slice strangely comes to more than the 18% charged over all. What could this possibly be? Without my giving actual figures, could this be possible?