Hey, there's something I don't understand about treasure maps.
 
I got the first one, that's just left of Stormway, called Granite Tunnel of Woe, killed the boss and got another treasure map. Now, when I check the statistic for Granite Tunnel of Woe, there's a part that says "Treasure's Plundered - Treasure Maps: 100%". This made me believe that I had got all the maps I could from this place. Now, the weird thing is that each time I rekill the boss at Granite Tunnel of Woe I get an entire new map, even though it says 100%. Can anyone explain to me why is that?

I believe that the % is actually the chances of getting those items. 
Therefore, you will get a Treasure Map 100% of the time, whenever you kill that map's boss.  However, there are still 2 more drops that you have not gotten from the boss.  These are usually 5% or 2% drops that consist of rare items.  Refer to a Hoimi table to get them. 
Or, just kill the boss 50 times and hopefully you'll get the items sooner than that.