The reason is that while the attacks were near simultaneous, Japanese raids and attacks on targets in Southeast Asia actually happened on December 8, due to time zone differences created by the International Date Line.

So, strictly speaking, the attacks on Pearl Harbor, while nearly concurrent with those in Malaya, the Philippines, Dutch East Indies, Hong Kong, Thailand, happened a day before as the history books record events, and Japanese texts would possibly record the start date of that phase of World War 2 as December 8 1941, due to the attacks all occurring on that day from a perspective in Tokyo.