what isn't clear? "at least" 5 years at 20GBytes of writes per day" seems pretty straightforward. They can't tell you exactly how long to the minute, unless someone can predict the future.
These numbers are all based on averages. If you have a million of them, a few are going to be dead on arrival, a few will die quickly, etc. MOST will last several years, some could last longer. There's too many usage factors for them to say exactly. So they come up with simple marketing terms like "at least" 5 years at 20GBytes of writes per day to give you a ballpark.