Reynolds numbers do make a difference, but it would be a *very* unusual airfoil that changed that much over such a small change in Re. The Jonkers published polar is peculiar in many ways.

I did reach out to both Ideflug and Jonkers about any data they had on tests, got acknowledgment of the requests but no response from either.

On Thursday, November 19, 2020 at 6:59:29 AM UTC-8, wrote:
On 11/19/2020 8:35 AM, Eric Greenwell wrote:
How odd the 63:1 only occurs at the high wing loading in 21M, and not at
the minimum wing loading, where the L/D is essentially the same for the
18M and 21M wings.
Not odd at all. Reynolds number changes make a big difference especially
with tiny chord. IIRC the original Ventus 15m increased LD 2 points with
ballast, and could not change wingloading as much as modern gliders.