I have always understood that there is a subtle difference:
"learned" is the past of the action of learning (e.g. Fred learned how to ride a bike)
"learnt" is the state that something (rather than someone) has been learned e.g. Mathematics was learnt.
The example with burned and burnt may be easier to explain:
The paper burned (i.e. it was on fire)
That piece of paper is burnt (i.e. the action of it being burned has caused it to be in a burnt state).
Happy to be told otherwise