HELP

Hello, I am a little confused. I watched the explanation video and the part that threw me off is number 8 going into the afternoon slot. I understand that rule six indicates the following: It cleaned Second, Fifth, and Eighth streets on afternoons. This is why I am a bit confused in the video the lady explaining put 8 in the afternoon and then lined up 1-8 and stated that we know 4,7,8 don't belong in the afternoon but while 8 was in the afternoon she crossed it out. I am very confused. How does 8 not belong in the afternoon when in video 8 is in the afternoon and rule 6 states that 8 must be in the afternoon. Please advise.