I am getting the full 9% (on buildings). Bear in mind that minutes are not displayed until the upgrade time is less than a day, so your 8.25% (let's say) might be an 8% reduction from eight houses excluding the remaining fraction of an hour that you cannot see. Some of my calculations for example show reductions of 9.09%, 9.23%, and 9.20%. I guess that if minutes were displayed then it would be 9.00% or very close to it.
Compare the upgrade time displayed before and after the building upgrade is started. If they are different then one or more houses are likely sitting outside the Burj's range and need to be moved closer. Conversely, if the before and after times are the same and the reduction is less than 9%, but the info window says 9% then there is a different bug with the math. I'm not saying that it isn't happening, only that I have not experienced it.