That is because your mesh isn’t ‘straight’ at the ends.
Like in this example:
The Simple Deform considers the ‘farthest away points’ and bends those 360° (your extremes ‘meet’.)
That is why I use the Knife tool to cut it straight.
From this video:
https://www.youtube.com/watch?v=-4eKbpt_ZtM&t=654s