Some triple-beat forms in Scandinavian Folk Music are characterized by non-isochronous beat durations: asymmetric beats. Theorists of folk music have suggested that the variability of rhythmic figures and asymmetric metre are fundamental to these forms. The aim of this study is to obtain a deeper understanding of the relationship between melodic structure and asymmetric metre by analysing semi-automatically annotated performances. Our study considers archive and contemporary recordings of fiddlers' different versions of the same musical pieces: polska tunes in a local Swedish tradition. Results show that asymmetric beat patterns are consistent between performances and that they correspond with structural features of rhythmic figures, such as the note density within beats. The present study goes beyond previous work by exploring the use of a state-of-the-art automatic music notation tool in a corpus study of Swedish traditional music, and by employing statistical methods for a comparative analysis of performances across different players.