Why do all analog pins measure the same value? I measure 2 analog pins: A4 and A5. The A5 pin is connected to another Arduino's digital out (through a 10k resistor) which has a simple program: 5ms on and 100ms off. The A4 pin is not connected anywhere.

The analog pins are inited with pinMode(APIN, INPUT), reading is simply performed with the analogRead(APIN) function (there is no delay between the readings). I have tried with delays between the measurements but the results are the same.

The results: 

The A5 pin (which is connected) measures the correct value. However, the unconnected A4 pin is clearly interferred by the input of A5. It works like this with another pins, delays between the readings, double use of analogRead()... I even swapped the Arduinos (Arduino Nano and Uno) and the results are the same.

Is it because of that these Arduinos are clones? Or are these Arduinos damaged?
If you want to measure a pin (i.e. defined as input pin, either digital or analog), you have to use a pull up or pull down resistor (either internal or external depending on what the MCU GPIOs have).

This resistor makes sure in case there is nothing attached, that the (mostly high ohm) resistor will make sure the nonconnected pin will either give a LOW or HIGH as input (or in case of analog, the minimum or maximum value).

When the pin is connected, the pull down/up resistor will change the result so marginally that it can be ignored.