I'm not so sure, bugs... It might be that the 109E pilots seem to know what they're doing more often. That might skew the results. If you look at the comparison charts, the spit1 outruns the 109, outclimbs it in several alt bands, out turns it, and outguns it (while the 20mm is powerful, the ballistics is bad, and once it's gone the 109E is nearly defenesless, while the spit1 has a steady punch throughout its ammo depletion).
The stats say the spit1 would win.
Comment on the spitII: The spitII was almost identical to the Ia, but had 20mm tested in positions in the wings. That's how I understood it. It was a stop-gap plane, not many made, and the SpitV was the real successor to the Ia