What causes overvoltage in power grid?
You are correct that the grid is finely tuned, however it is not so static as that would lead you to believe. The entire grid is an immense machine that is quite unstable. Constant monitoring and re-adjustment is required so that the system maintains stable operations.

While you are correct that a generator generates a stable voltage (for the most part) the load on the grid changes each second. Systems monitoring these changes cannot always react instantaneously, especially when big moving object such as generators are involved.

Lets start at your home. The transformer supplying your area has three phases. The city/town planner would have devised the houses in your neighbourhood into (almost) equal amount on each phase. Now if the loads differ, it will cause minor shifts in the voltages in each phase as the phases become unbalanced. This is usually minor but can easily cause the minor fluctuations you see. If you can graph measurements over time, it should be interesting how the fluctuations look during peak times (mornings and evenings).

There are many other ways in which the grid is dynamic: Transmission lines heat up and cool down changing their resistances, solar activity induces currents in transmission lines, entire towns get knocked off the grid due to an accident. My personal favourite instability is generator phase. Generators need to be kept in phase and at frequency, however when the load on them (the grid) changes, it causes the generator to slightly speed up or slow down. This is compensated by with reaction wheels that release and absorb energy from the generator.

All of the above changes the load on the grid and therefore you will see voltage fluctuations.