It's a visual effect . It began leaning when the third floor was done, and later the architects tried to compesate it, so they built the rest of the floors with one side taller than the other. So if you took the photo from the third floor to the upper one, the tower doesn't seems like is leaning .
Of course it seems not to lean if u take a pic from a floor to the next one... ur reference frame is wrong.it's be so in any case coz brain tends to "build" references...anyway, sorry for being boring The shot is good,as the frame is, but for who doesnt know the leaning tower this pic doesnt communicate its essence which is, in the matter of facts,leaning. Regards.