I don't have an answer for that, but I saw in the past posts talking that when they used to train for hours online with summoned monks and some weak mob to skill the shielding, some creatures would progressively hit a little bit harder as the time passes (according to what I read). But I can't guarantee the information, since I was never really fond of training online back when it was necessary. I think that I read that info on Tibiabr forum several years ago, haha.