Year-to-date %change 點解 ?

今年截至本月(或日)與去年同期之改變(以百份比計). For example:
1. Today is 30th September 2006.
2. 1-9/2005 profit is HK$5,000,000
3. 1-9/2006 profit (i.e. YTD profit) is HK$6,000,000
4. Year-to-date %change = (6,000,000 - 5,000,000) / 5,000,000 = +20%