I assume it's due to the slanted nature of the descender of the 'y' character meaning there appears to be a lot of free space on the bottom right part of the black 'sky' box which you do not get on the left with the 's', perhaps tricking you into seeing what you perceive as an asymmetry. Don't feel 'shot down' by me pointing out that your assumption was incorrect . You said you felt the logo looked off-centre and I didn't think it did, so I checked. There's no malice in my comment, merely trying to give the most accurate/useful possible feedback to the creator of the mock.
Guess who's big in the back time!