How Frequent Do Users Tweet Or Check Updates?

With so many Twitter apps present now, you would expect lots and lots of people to be tweeting every move that they make. But hardly 27% of the people write their Twitter updates on a daily basis. Here is how frequent people tweet:

Daily : 27%
2-6 times per week : 17%
Once a week : 8%
Less than once a week : 26%
Registered but not tweeted : 9%
Don’t tweet anymore : 15%

Here is how frequently people check for updates from those they follow:

Daily : 46%
2-6 times per week : 10%
Once a week : 9%
Less than once a week : 19%
Registered but not checked : 5%
Don’t check Twitter anymore : 12%

P.S: The numbers don’t add up to 100% and I wonder why.