Just do the maths to figure out why. 99.999% of 1 year is 365*0.99999 which is 364 days, 23 hours 54 minutes and 44.6 seconds. So if your server reboot takes longer than 5 minutes, you have already run out of time, even in a year. I have created a table with the allowable downtime for various percentages:
|%||Per Year||Per Month|
|99.999%||5 mins||25 sec|
|99.99%||52 mins||4 min 22 sec|
|99.9%||8 hrs 45 mins||43 mins|
|99%||3 days 15.6 hrs||7 hrs 18 mins|
|95%||18 days 6 hours||36.5 hours|
Percentages are often used to hide unpleasant facts. For example, a condom that is 95% effective means that if you have sex more than 20 times, you might as well not bother, because 95% means 19 times out of 20 you're protected, and the twentieth time it will fail, if not sooner.