This is because of the capacitance between the cores in the cable.
Capacitance between cable cores will only have an effect if the test was done with AC, in which case the leakage test would be meaningless. Not only that, the higher the frequency used for the test, the greater the "leakage" would appear to be, when in fact the actual resistance is considerably higher.
Insulation testing is done using DC to avoid this error.
It is important to differentiate between capacitive reactance (which exists only on AC) and resistance which shows on both AC and DC.
For example, a 100m roll of three core flex has about 6nF between any two of the conductors. At 50 cycles, a capacitance of 6nF has a reactance of about 530K ohms. One would assume poor insulation if this was the DC resistance.
The modern Megger tester is essentially a high voltage DC inverter with a calibrated high output resistance. The output is applied to the device under test and the resulting resistance reading is determined by measuring the voltage across the D.U.T. in the same way a digital multimeter works on its resistance ranges. The high output resistance is required also to protect the D.U.T when stress testing. Basically, you can put very high voltages into things without damaging them provided the current is kept very low.
