I think it likely that a lot of the 2016 additions are as you say, just re-registrations. However, I don't understand what you mean when you say that 200 RV-6s were deregistered since 2010? Your graph shows 51, so I guess the numbers for each year is just an end sum. An example would be 60 6s struck off the registry in 2013, but 20 new planes added. Is this correct?
The numbers shown are the net increase/decrease for each year over the preceding year. So for the RV-6 in 2013, "-40" means that forty more airplanes were removed from the registry than added.
The FAA Registration database includes a list of all de-registrations. My "200 RV-6s deregistered since 2010" were based on the January 2018 database, and counted all the RV-6s that were deregistered since January 1, 2010.
It should be noted that the number of deregistrations in a given year may vary in later years. For example, the January 2018 database shows 48 RV-6s deregistered in 2013, while the January 2016 database says 54. This is undoubtedly cases where aircraft were deregistered in error (usually by an owner's lapse) and restored to the list.
Also, don't people obtain their registrations prior to actually completing the plane, or are your numbers only showing actually flying aircraft?
My numbers are the total registered aircraft. There are no independent sources on the number of aircraft actually flying. The FAA says 40% of all registered homebuilts are inactive, which would include both not-yet-flying and abandoned aircraft.
In addition, each registration entry includes a code for Certification category. Supposedly, aircraft which do NOT have entries in this field have not yet been awarded an Airworthiness Certificate. At the end of 2017, for example, there were 1,972 total RV-6s in the registry, but only 1,796 of them were listed as having Experimental Amateur-Built airworthiness certificates. Ten additional aircraft are listed as certified in other categories.
However, this field is not that reliable. For instance,there are almost 2,400 Cessnas that have blank entries in this field. According to the registry, there's one normal-category and one Experimental Light Sport RV6.....
Ron Wanttaja