took about 4 seconds to return to the original value near 300
A charge is slowly building up until it gets to a steady state value. The i/p is a high impedance/resistance so noise in the surrounding circuit is effect your readings.
When you write your sketches you initialize variables so you don't have undefined values in them, hence you avoid bugs or errors.
When you hook up to i/ps and o/ps you connect hardware to them so they are in a way initialized i.e. they aren't at some unknown level of voltage. These can be external pull up or pull down resistors, internal pull up resistors, sensors, switches etc.