Are you sure you weren't mistaken (I am sure you were)? There are a number of things which could have caused this mistake. All or any of the following might be true.
1) You confused the boundary of your own line of vision with the vertical aspect of the horizon. Since you can see a certain distance, if you look around from a single point, a circular locus of vision is formed. Normally it's easy to see the difference, but I'm sure you had a lot more on your mind as the pilot of a fighter jet.
Right, but at ground don't have the same view, doing the pre flight check sitting in the cockpit.
2) Your window was curved and distorted the horizon
Is the same window in the plane in the ground, also the same at lower altitude, and the view of the horizon in the ground is totally different to the one I saw at 60.000 fts above the earth.
3) You expected curvature, and since you were focussed on flying the plane, your mind is just filling in the blanks since you had more important things to worry about.
At that high, was only for test new improvements to Dassault Mirage Pantera, we use Kfir aeronauticals instruments to improve the old Mirage 5, I had a plenty of time to look out of the cockpit.
4) The effects of high-velocity flying impaired your visual reasoning somewhat
Is the same velocity that I can get at lower altitude, the effect is not the same, at lower altitude the earth looks different.
5) You might just be lying, but I will assume you aren't (unless you adamantly deny the above four possibilities)
All is posible, I am a big fat liar. who don't live in the southern hemisphere.