For what it's worth: LiitoKala/Colaier Lii-500 inconsistent IR measurements FIXED
I take great care to minimize variation and even then it occurs to some extent (check here for example), this is normal. Contact resistances are normally high by themselves even when the surfaces are clean and tightly coupled to, let alone if they're not.
The order of magnitude of a cell's internal resistance is close to that of a contact, expecting to measure it this way is rather tall order. 
Cheers ^:)