Hi mir4ea, it's because one curve is a closed curve but the second one is an open curve. When you try to loft between an open curve like that it will not do the kind of "seam matching" that is done to align 2 closed curves with one another - instead just the natural start and end point of the closed curve is matched to the start or end of the open curve.

You can tell if a curve is closed or not by selecting it and then looking at the properties panel, the type indicator there (on the right-hand side of the object name on the top of the properties panel) will say "Closed crv" if the curve is closed or just plain "Curve" if it is open.

If you zoom in on this corner area of the open one you'll see the pieces that don't match up which is why it's not a closed curve:





Hope this helps!

- Michael