I have been taking a closer look at HBCU 4 year institutions and am looking at their 6 and 8 year completion rates (C200_4 & C150_4). I have noticed that several of the 8 year completion rates are lower than the 6 year rates. Why would this be true? Shouldn't the 8 year completion rates always be higher?
I don't know about the exact schools or years that you are looking at, but in general this situation can easily if the 6-year graduation rate is based on looking at the students who entered in e.g. the fall of 2013 and graduated by 2019; and if the 8-year graduation rate is based on the students who entered in fall of 2011 and graduated by 2019.
Those are exactly the entering and graduating classes that every college submitted in their IPEDS surveys last year. If you're looking at the latest data in the College Scorecard you're probably looking at those years.
And as the example shows, those are two completely different groups of students! Maybe the students who entered in the fall of 2011 didn't do so well and had a low graduation rate whereas the ones who entered in the fall of 2013 enjoyed campus with better support and programs ... or maybe the fall 2013 students were just better.
And thus the 6-year graduation rate of the fall 2013 cohort could be higher than the 8-year graduation rate of the fall 2011 cohort.
To add to what Mike Tamada said, depending on the school you will only see minimal improvement in grad rates between years 6 and 8 as they approach what will be the final completion rate (as in even if you ran it out to 20 years, the % after year 6 isn't going to change much). When I've looked at data there is a big jump between years 4 and 5, then not much 5 to 6 and probably even less above 6.