What is reason for 6.6% levy?

Without seeing exactly what has been charged, it is hard to say precisely what this relates to but a zero income tax bill does not necessarily mean a zero bill for social charges (which are shown on the same avis d’imposition). Social charges may be levied on many kinds of income, including savings and investments apart from exempt ones such as Livret A.

Some financial advisors believe there is evidence that changes are being considered relating to how social charges are applied (revolving around differentiating between part of them that is considered to relate to the French social security system and part that is not). The change might relate to this, however most tax offices did not apply such changes in 2017.

A figure of 6.6% is the rate at which one of the charges, CSG, is typically levied on pensions. Note that you should not be paying that on British pensions if you are a UK state pensioner with an S1 form for your healthcare.