In 1987, the average time-served for 2nd degree murder in Florida was 6.8 years [state.fl.us]. That is more than five, but that is the average, so many murders served less time.
Do you know how to read a graph?
If you look at the page you link to, you will see that the 6,8 year average for second degree murder was twenty years ago. The average by 2003 climbed to 21,6 years and the average murderer close to 27 years.
Now, since your original statement was in the present tense I'm still going to have to go ahead and call bullshit. The average SECOND DEGREE murder convictee will spend 21.6 years in the joint as of 2003, and the sentences were on the rise at that time. With all the mandatory sentencing laws going around, I will bet you that the average is higher today, ten years later.
Also, remember we're talking about Florida. If you're white and you kill a black guy in Florida, and you pay a $25 fine and get two tickets to a Marlins game and a coupon for more ammo.
So, there's still no indication that "many murderer spend less than 5 years in jail". You wanna hit up the Google and take another shot at it? I've got nothing to do tonight.