It is not surprising, then, that the interval space first expressed is C5-G5, that the space becomes D5-G5 as the tune goes on, or that the entire song is focused on ^8 in its upper voice. See below.
In the variation, van Eyck expands the space upwards and downwards to the octave A4-A5 -- see the box in the figure below -- which leads nicely into a broken two-voice texture in the cadence that ultimately gives us a complete octave space ^1-^8 in the final bar as well.
Reference for the title: Jacob van Eyck, Der Fluyten Lust-hof, edited by Winfried Michel and Hermien Teske (Winterthur: Amadeus Verlag, 1984). Michel and Teske (vol. 2, 57) mistakenly assume that the piece is in C major and ends on the dominant.