A very interesting topic
A couple of things to think about you are using a R4xxx refrigerant so what abiut glide
Also you are see a solid head of liquid in your reciever so stratified layers can allow for differences in liquid temp at the outlet the bottom and the surface area also consider the possibility of vapour super cooling
Also your temp measurements are likely to be the cooler if measured on the surface of the pipe influenced by location ambient
Many factors come into play with a free drain cond and how it enters the reciever