How the bands actually work
Most modern scales use a mean of 100 and a standard deviation of 15. That puts roughly 68% of people between 85 and 115, about 95% between 70 and 130, and the remaining tails at the edges.
A score of 120 is not '20% smarter' than 100. It means you scored higher than about 91% of the reference population on that specific set of tasks, under those specific conditions.
Why timing and test conditions move your number
Speeded tests reward pattern recognition under pressure. Sleep, caffeine, screen size and whether you were interrupted can shift a result by several points in either direction.
That is why a single run is a snapshot. Taking the same style of test on three different days gives you a far more honest range than any one score.
What an online test can and cannot tell you
A free online test can show you which reasoning categories you handle fastest — number, logic, verbal, pattern or spatial — and how that profile changes when you are tired or rushed.
It cannot substitute for a supervised clinical assessment, and no legitimate test will diagnose anything from twenty multiple-choice items.