It is quite simple “I’m 99% certain I’ll accept the offer”.
Having recruited for employers all over the world for many years, I can tell you that I know exactly what that phrase means. I have heard it perhaps 300 times, and on the balance of probabilities that would mean that 297 of that 300 have accepted the job.
Except in reality they never do. Because a candidate who is 99% in, is 100% out.
A bit like a football really. A ball that is 99% over the goal line is 100% not a goal. On the flipside a tennis ball that is 99% out is 100% in.
So 99% means to me that they are preparing you for the 1% doubt. That they quite like the idea of the job but they know they won’t accept it. But they don’t have the guts to tell you today, so they’ll leave that lingering doubt in your mind overnight until they call you tomorrow. Or go quiet on you.
If you want to reassure a recruiter that you are going to accept their job (but you possibly have no intention of doing so because you are seeing three other employers in the meantime) then do not use the 99% word. They have heard it all before.
In fact we are much happier with lower percentages, because they are much more genuine and quite often end in a good outcome. Somebody who is 90% certain may well accept, and someone who is 80/20 will accept most of the time.
So I’ll never believe 99%. But I might give you 110% for effort.