A lot of online IQ tests aren't accurate. They're there to bring people to their websites, and possibly look at ads. Or, they may want to charge you for results that don't mean much, even though test takers might think the results mean something.
That doesn't explain your low result, tho. Most of those inaccurate tests err on the high side. If they give you a high score, you are more likely to buy their vanity certificate, and more likely to suggest others visit their website.
It could be, when you got the score of 100, you were simply having an off day. It happens. It could be it was a poorly designed test. That happens, too. Some days you do poorly. Some days, you do better than you usually do.
I'd go with the results that allowed you to join the High IQ society.