Why do you assume it is solid fact that every encounter with an enemy fighter will result in you winning just because you have uber-fighter stats?
My lowly UK & RM T5 fighters do pretty good against all nations and levels. I have no idea how many vets or their ability is on the other side, but I've seen plenty of times when my UK fighters tear apart KM, US and IJN fighters with ease. My RM fighters are the same...but as in all encounters, they too get shot down. Not always by uber-fighter pilots, but even lowly T4s and T3s.
A couple of Tune-ups may be the difference too.
A tactic you may not be familiar with is multi-pilot formations. When in groupings, a lead pilot group might be the intended target for your fighters, but my other fighters are looking to get your planes at the same time your trying to intercept the 1st squadron. A little maneuvering can make the difference. If you're not engaging your FPs, you can get eaten quickly.