Why do 1 Chronicles 21 and 2 Samuel 24 give different numbers?

2 Samuel tells of the same census, and in three places its numbers are not the ones in 1 Chronicles. The total: "All those of Israel were one million one hundred thousand men who drew a sword; and in Judah were four hundred seventy thousand men who drew a sword" (21:5), and "there were in Israel eight hundred thousand valiant men who drew the sword, and the men of Judah were five hundred thousand men" (2 Samuel 24:9). The famine: "either three years of famine" (21:12), and "Shall seven years of famine come to you in your land?" (2 Samuel 24:13). The price: "So David gave to Ornan six hundred shekels of gold by weight for the place" (21:25), and "So David bought the threshing floor and the oxen for fifty shekels of silver" (2 Samuel 24:24). Neither book explains the difference, and these notes don't either. 2 Samuel 24's honest question sets out the same three. Much else is told alike in both: three choices (1 Chronicles 21:10-12; 2 Samuel 24:12-13), seventy thousand dead (1 Chronicles 21:14; 2 Samuel 24:15), the LORD's "It is enough. Now withdraw your hand" (1 Chronicles 21:15; 2 Samuel 24:16), David's "these sheep, what have they done?" (1 Chronicles 21:17; 2 Samuel 24:17), and an altar on a threshing floor bought for a price (1 Chronicles 21:24-26; 2 Samuel 24:24-25).