Glo Book cover not fullscreen?

Hi, the problem I'm facing is illustrated in the photos I took:


The left one is the first page, and the second is when the device is in sleep mode.

I'm using calibre for epub->kepub conversion and I'm wondering why is there empty space on left and right? This happens to all my ebooks.

I'm using Kobo Glo HD with the latest firmware.


Thank you in advance for any help!