Bike measurements

There seems to be a glut of bike questions around here right now. I thought I might add one of my own. I bought a bike last January, and the sticker on the bike said 58cm. From everything that I'm reading, it appears that a 58-frame bike is massive for little 5'10" me. When I was in a shop here in Germany recently, I commented on the size of the bike, and how it sure seemed odd that it fit me okay - to which the shop owner replied that my bike was the same size as his, and we're about the same size, so 56 sounds about right. But wait, I said, my bike is a 58. He measured the thing, and sure enough it's a 56. I'm now stuck wondering whether Europeans and North Americans measure bikes in different places, or whether I should be wary of advice from this particular shop in the future! Any ideas?<br><br>
Thanks!<br><br>
Bradley