“Learned” and “Learnt” mean the same thing. The “t” ending is from the older English spelling while the “ed” ending is from the newer American spelling. Either are valid. They are both past tense of the regular form.
Another example is “dreamed” and “dreamt”. Verbs have a tendency to shift spelling to become more regular.
Which do you prefer? I shall be using the “t” ending exclusively from now on. Just because I can.