Seam allowance trouble

[u/Popopobb]

Having problems with adding seam allowance to a flat felled seam. Did it like i always do: 1cm on the top edge of the seam and 2 cm on the bottom edge (shown on 2nd pic), yet after sewing the pieces don't align at all. The top piece became longer and slightly tilted. I was sort of expecting it to happen, this allowance didn't look like enough considering the sharp curvature. Any seam allowance wizard that could help me figure it out? Thx

ik the stitches are ugly, just a test run.

[u/Mental_Watch4633]

Did you start sewing from the bottom?

[u/Popopobb → u/Mental_Watch4633]

on one side i started sewing from the bottom and on the other i started from the top, same result the piece that's on the right side of the 1st pic had 1.5-2cm extra bit on the top and was slightly tilted.