The first is the coupling (mainly capacitive) between the primary and secondary windings on the transformer.
Having a high input impedance means it only takes very little capacitive coupling to get a reading.
However, there is enough capacitive reactance to cause a significant phase shift in the current that flows through the load.
At this frequency a parallel resonant circuit is formed between the primary inductance and the net capacitive reactance reflected back from the secondary.
The capacitive reactance Xc becomes very large and the inductive reactance XL becomes insignificant.