I've only just realized this is the lemmit bot reposting this from Reddit -.- I'll keep it up for others.
Voltage does fluctuate often throught the day, and even throughout the hour. It's partly from your electrical provider and partly from people using the electricity in your neighbourhood. My particular voltage will go be between 115 volts and 122 volts in a day.
You can think of the voltage like the water in a storage tank or water tower.
When you open a tap, it uses a bit of that water so the pressure drops a little. The supply company will try and maintain the same level of water throughout the day but many people are opening and closing taps which changes the water level a bit.
It looks like the voltage drops in the middle of the day which is when people would be using electricity the most.