It doesn't always happen. A 7805 will give an output voltage close to 5V. but some will give you a voltage slightly less and some slightly more than that. It has to do with how they are fabricated etc.
Note: if you test several 7805 and they all give a voltage slightly lower than 5V there is either something wrong with your circuit, or the voltmeter you are using.
They are not precision, they vary widely. 8.8 for a 9 sounds pretty good to me!!! The only strange thing is they both go below. Are you drawing the max current? Do it get hot? You have heat sink on it? Make sure you have at least 3V head room.( for 7805, the input need to be something like 8V, read the spec to be sure.)
I had seen those regulator get hot even under light load. I am not exactly a fan of these regulators.
4.9 volts instead of 5.000 is only a 2% error and these things are produced in millions for a few cents each.
It just reflects the difficulty of accurate doping of materials, particularly for the voltage reference inside these ICs.
It is the same reason that transistor gain varies so much for the one type of transistor.
A 2% error isn't going to make any difference for many circuits (especially digital ones).
If it will make a difference, you can get better regulators or variable regulators.
I have tried to trace oscillator drift and found it was due to the change in voltage output of these regulators with temperature. They need a heatsink, but seldom get one.