To be honest, I am of the opinion that there is a clear distinct between the two.
I think the difference between phrasing a question as 'may I' and using 'can I' instead, is that the first seeks the permission of the person to whom the question will be addressed to allow the question to be asked.
In other words, you are requesting their attention, space, time - you are not assuming - or presuming - that you have a right to ask the question or receive an answer. To ask a question thus is signalling recognition of this.
The second is self evident. Anyone who has mastered speech, or writing 'can' - or, is able to - ask a question.
Clearly, when people are asking if they can ask a question, they are seeking permission.