Optical illusion or lens distortion?

Neither one on my monitor.

The concrete decking in the middle doesn't look skewed to you? The balconies on the left look straight and horizontal, whilst the middle ones look warped on the left side.

Different angle here:

Nope, I don't have that critical an eye, sorry.

It is not a case of critical eye. Have a look at the next image I just posted. Seems to stand out regardless.

its how the building has been designed and built.

Gotta agree there - I'm pretty sure that's just how those balconies are designed. Otherwise the walls of the building would look like that as well, don't you think?

also the bottom lower levels have the same curved balconies in the middle of the building.

Compare the top two balconies in both the photos. Why the discrepancy then?

I don't see any discrepancy...

This is the way I think that building looks:

The outside walls of the building are straight, the balconies are curved (even more so than my sketch shows, because they seem to be on outer sides as well...

Of course there might be some lens distortion as well, but I don't think it's significant.

Those balconies look different again from this angle:

While there‘s more distortion here at play, I still see the same thing. Straight edges on the left and curved balconies on the right. Doesn‘t look confusing to me.

Everything (relatively 😉, didn't put too much effort in it...) nice and straight, except those balconies on the right...

EDIT to add: It's not impossible that the building is indeed getting wider as it goes up, but I doubt it, looking at the other images.

Reminds me of this:
www.deceptology.com/deceptive/the-horizontal-lines-illusion-why-straight-lines-appear-curved-when-placed-next-to-each-other.html