join into untrimmed surface

hi there,
iam trying to make a shape like you see in pic 01 (shape.jpg)

it is two arcs on the bottom and one arc and a half of an ellipse on top. when i did this the first time it worked out fine. (firsttrywork.jpg)

th lofts gave me untrimmed surfaces which i could join and cap later on.
in the second set up i had a more or less identical procedure, but the one loft would always result in a brep, which i didn´t want to have (dontwork.jpg)


since i couldn´t solve this problem, i made the longer way and trimmed a full circle loft with the other shape.

can anyone help me understanding this problem? how does a loft result in an untrimmed srf/brep? s.th. about datatrees?

pls hlp. thx,

phillip