This is a total guess, but it may be because people are getting calls for immediate interviews and may not have enough time to fully prep. I studied for 2 months for my interview, any less than that and I would have struggled on the tech and cog tests. Some of my buddies were able to cram study in 2 weeks but they had a lot more free time to do so. With family/work/mil commitments, most of the people I talk to just don't have the ability or time to cram.