Same same, but different

Yes, I know that most people who have ever visited Thailand know this phrase: "Same-same, but different". But it still puzzles me why they say it. They say it even when comparing a hotel room of 6 sqm with an bungalow of 25 sqm. Or when asking whether to order two completely meals (like Thai curry and Pizza). I mean, if it is different then WHY on earth use the term "Same same"? In Thailand you can even buy T-shirts saying on the front "Same same" and on the back "But different".