I have created an elbow (SLDPRT209899.SLDPRT) according to the real model size. It's length are not the same in horizontal side and vertical side. When I use it to create a piping assembly(with pipe “SLDPRT77.sldprt”,config “DN40”), I got the wrong pipe. If I changed the long side of the elbow to 2mm shorter, then the pipe shape would became normal. 

Can somebody tell me why this happen?

Is it because the elbow is not the same in length of horizontal side and vertical side?

Is there any way to create the right pipe without change the size of elbow?

Pls see my attached captured screen, and parts for reference. Thanks.

Kico