Because every machine will have a bit different needle placement. The foot helps, but you need to figure out the scant 1/4" for each machine.

Great tip! Thanks!

Because they are a mechanical manufactured product which most likely is not checked for its accuracy. This is the nature of manufacturing. One must know how the equipment works. This is part of "knowing your machine" Even the machine might not be centered correctly. One must adjust to these differences. Are all measurng spoons exactly 1/4 teaspoon. I could go on and on.

I tell students they must see the fuzz on the bottom layer to be sure they are lined up exactly. If they can't see it then most likely the bottom layer will be out of alignment.

I have never had a problem sewing 1/4 with any of my brothers and their 1/4 inch foot, for scant you can just use your regular presser foot and adjust needle position to get your scant. I haven't needed a scant yet but I have not worked with anything but strips,applique, and PP, no triangle or set in seams yet that need that scant but it can be done, you just have to measure on your machine and write down your settings. I don't think its the feet. I find the ones without the lip more accurate for me as you can see you fabric to make sure it guides straight. Mine always measure 1/4 in with my ruler when I do my cutting

It also pays to double check when you change fabric type, like going from cotton to flannel. The thread weight also plays a part. The thickness of the thread effects how much room will be taken up when you press your seam.

Tips like this is what makes this such a great place, especially for a new quilter like me. Thanks.

Ricky Timms advises this and when he sews curves it works like magic i've tried it and I can make it work !!!

Thank you I have got to try this....

I don't either. On my machine, the needle is slightly more than 1/4" away from the edge of the so-called 1/4" foot. Therefore, when I start up the machine, I have to move the needle to the right 2 positions to get right measurement.