If your fabrics ARE cut to exactly the same lengths, but your seams are coming out uneven, it is a matter of how you are handling your fabric. No, you do not need to pin nor is a special foot required. Understand this concept though. Your feed dogs are grabbing the fabric towards your machine at the same time that your pressure foot is pushing it away......which causes it to feed in unevenly.
Notice in my picture that my hand appears to be curled up towards me. This is providing the resistance to the feed dogs, and will not allow it to feed in.
This is one of the tricks of the trade in industrial sewing. A technique that works, not only for straight stitching, but curves as well.
I saw some posts where you were being advised how to feed in the bottom, but I really don't think that is the solution you were looking for. You were asking "why" this happens.
How to hold your hand to give even feed.
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