First, a couple caveats: The Vit Plant is still under construction forces, so none of my experiences apply to their positions. My understanding is that the initial test used a couple years ago got one of the companies into a lawsuit (rumored allegations of the test being discriminatory and/or who did or did not have access to study groups) so no test was going to be used for hiring this time around. That is all rumor however.
The trainee RCT/HPT hiring process as of a couple years ago was:
1. Resume submission
2. In person 30 question multiple choice test based on the first FOUR chapters of the DOE Core Academics
3. Short interview mostly from an approved questions bank
4. If hired, ~3 months of classroom training to cover both Core Academics and Site Academics with 3 chances to pass a test on each book, and an oral board that must be passed within 6 months of hiring
Those hired NOT as trainees IIRC had the same hiring process but then had a couple weeks to study before testing, no instructor lead training. Keep in mind they can always change the hiring process. Back in 2009-2010 trainees had to pay for the instruction course for the DOE Core Academics, and would only have that money refunded if they passed.
So as to why people failed the 30 question test or even the Core Academics test at a "high" rate? In my opinion it's a variety of reasons. Most people are bad at even basic math, even after being taught it. Test anxiety is certainly a factor. If someone doesn't have experience with how the DOE writes questions based on Terminal Objectives (TO) and Learning Objectives (LO), they probably will not study correctly even if given all the material. There are some LOs where it seems like there would be a bunch of questions based on the amount of material in the study guide, but in reality only one or two questions actually answer the LO and most of the material is fluff.
All this is opinion and things can always change, so take it with a grain of salt.