In the USA , 120 VAC devices are actually rated 125 VAC . Same , 240 -> 250 VAC . I gave actually seen 255 VAC . Some of this depends on if you ate close to the " end of the line " or the head of the line " , of a run of primary ?
Some transformers have or had taps , the power company could adjust the secondary voltage , to some extent ?
I have also seen voltage vary with time of day , with load . In the summer , around 05:00 PM , when A/C's get turned to max , electric ranges , washing machines , clothes dryers , water heaters and every thing in the house gets turned on .
In the US if you have had to replace equipment due to overvoltage, you can make the power company put a voltage monitor on your line, this is a portable device that records voltage levels for a month or so. This will also record spikes and so on. If an isolation transformer solved it then very likely there's distribution problems. If your in the US and the power company balks then threaten to go to the PUC as all of them are regulated. My father sold high voltage power distribution gear for all his life and quite often power companies find things like failing step down transformers in their distribution systems due to industrial users complaining.