Learned vs. learnt

These two frequency lists of this pair of words come from the NMC and BNC. Can you think which is which? And why?

Given that both the BNC and NMC are about the same size and both British-based texts, the major factor that does differ is the time when the two corpora were compiled. The BNC containst texts up to about 1996 and the NMC from about 2005 to 2008 (I think).

Does this help you decide which data set belongs to which corpus? For the answer, click here.