"The elevation in the alanine transferase (ALT) occurs when there is damage to liver cells. This can happen for a number of reasons. Medication reactions, toxins, cancer and hepatic lipidosis are probably among the most common causes in cats but there are also lots of times when the ALT rises for no apparent reason and then is normal on testing after a week to a month or so"

Found this on a vet info site. It would suggest in the absence of other abnormal bloods or symptoms either the raised ALT is idiopathic or perhaps as kozykatz suggests error in the test.