Having taken sample tests online and now having taken the genuine article, there are some major differences.
1. You can't use a timer.
Online tests count you down from 12, and you can see it. Wonderlic doesn't let you see a clock and offers only a one-minute warning. You can't use a calculator either, only a couple of pieces of blank paper and a pencil.
2. Many questions are not multiple choice.
On the sample tests I had taken, many of the math questions were multiple choice. This enabled you to do a quick glance at the choices and move on quickly. Most of the math questions on my test, and the one given to the players, are fill-ins, which means you can't just take your chances guessing.
3. Not feeling pressure of time takes practice.
Whether they admit to it or not, many players study for the Wonderlic. It's a factor because teams still administer it, and some teams care about low scores or are impressed by higher scores. I took two practice tests and clearly was affected by the clock winding down. After the one-minute warning was given, I felt like I was essentially rendered useless.