A point in space doesn't have unique values for the electric and magnetic field strengths. Why is this?

Answer

Electromagnetic waves can alter the electric and magnetic fields, and so the values of these fields at a point are not unique.

A redefinition of the origin of a reference frame will change the values of the electric and magnetic fields at a point.

Within the same reference frame, a magnetic field can be reexpressed in terms of an electric field.

Whether a field is seen as $$electric$$ or $$magnetic$$ depends on the motion of the reference frame relative to the sources of the field.