Learnt and learned are both used as the past participle and past tense of the verb to learn. Learned is the generally accepted spelling in the United States and Canada, while the rest of the English-speaking world seems to prefer learnt.

Another spelling variation of the word "while" used in the United States and Canada; however, the rest of the English-speaking world seems to use the word "whilst".

For the difference in spellings, we can blame Noah Webster of Webster's Dictionary fame, who, in 1789, spearheaded the push toward "American" spelling variations on some words.