Help With Koreader formatting epub

[Fistandantilus]

Hello everyone!
This is my forst experience with a BOOX device (in my case a Palma 2) and KOReader. I'm very satisfied but I have minor problem I cannot address. I have an epub file (i would say correctly formatted) but text with "center" attribute is not perfectly centered on screen.

I know, I KNOW, this a small thing but it bothers me nonetheless... and it has become a challenge.

File_1 and 2: As you can from the image the first line of centered text is slightly shifted to the right. This is the book opened in KOReader.
File_3 is the xml open in SIGIL.

This DOES NOT happen on the same device using the native BOOX app (see File_4 and 5).

I could upload the CSS file for you to see... but I do not think is a formatting issue (the preview in SIGIL is actually correct).

Again... is a very silly issue, I know this, but know I would like to know why this happens.

Thank you.
Luca

PS: center text by h1 class is actually correctly centered.