You need to read up on pixel aspect ratios and how it affects video. You have chosen an encoding preset that uses anamorphic pixels and since computers use square one it displays wrongly.
Your composition was rectangular pixels, your artwork was improperly interpreted, you failed to understand pixel aspect ratios. Generally you always want to create all of your projects using After Effects default composition sizes that result in square pixel projects. Only the very rare project would be delivered in a non square pixel format.
If you post your composition settings we can verify this is true and that I'm not just guessing. The only other option is that for some reason you adjusted the output module and render settings to scale the project and produce a non square pixel render.
Thank you, Mylenium and Rich!
Appreciate you guys' help so much! Forgive my lack of knowledge in pixel aspect ratio. But with your kindly feedbacks, now I know how to fix my problem.
Again, thank you very much!