In my understanding also, simple way to remember is:
Variability means - Inconsistent ( So Variability risk means key characteristics of planned event to be uncertain- The Planned event here is "productivity", it will be there but will it be above target or below target or just the target ).
Ambiguity means - Inexactness or fluctuation which is kind of similar to inconsistent but here it the key word is "Imperfect knowledge " . Example - Future Development in Audit process.
I strongly suggest reading this PMI article about risks, it does a good job explaining various types of risks:
Variability risk: There is a set number of possible outcomes but we don’t know which one will actually occur.
(ex. The number of errors found during testing may be higher or lower than expected)
Ambiguity risk: Come from uncertainties arising from lack of knowledge or understanding.
(ex. New disruptive technologies or market conditions)