Input impedance of differential amplifier
Everything below assumes ideal Op. Amp. inputs: zero input currents and zero differential voltages.

In the first circuit, when you vary V1 keeping V2 fixed, the voltage on the inverting input of the ideal Amp. Op. is fixed. For this reason, the feedback resistor is not relevant. A small delta V1 leads to a change in the input current that depends only on R1, since its other terminal is at a fixed voltage. When V2 varies, since there is no input current at the Amp. Op., the input current is determined by the equivalent resistance: R1 + R2.

In the second circuit, when only the differential voltage changes, regardless the actual absolute voltage on the input pins of the ideal Amp. Op., the terminals of the R1 resistors connected to the inputs are at the same potential. For this reason, a delta Vdiff results in input currents as if the R1 resistors were connected in series.