Analogue ports behaving differently

I am using this circuit: http://www.fiz-ix.com/wp-content/uploads/2013/04/IRPhotodiode.png

I am using visible light photodiode instead.

On A1 or A2 the voltage is pulled up to about 4.7v even though the junction of the diode and resistor is only 1.2v when the port is disconnected. However A3 does not have this pull-up effect and exhibits a high impedence so that I can read the voltage at the junction of diode and resistor. Any reason why A1 and A3 behave differently?