As described by Ray Cracknell G2AHU when he reviewed the results of the first year of the tests (Six and Ten Report, December 1993), the monthly differences are surprisingly small and could be a random effect. However, looking at the monthly data averaged over the last 4 years, one might be able to conclude that summer and autumn provide the best conditions for tropo-scatter over this path.
September and October are picked by G4ASR in his generalised "Dxers Year Planner" (The VHF/UHF DX Book) as best months for VHF tropo. The 50 MHz test data seem to confirm this. Due to the frequency of settled weather patterns perhaps?