Originally Posted by L'il Chickadee

Welcome, Tabinfla!
Sounds like you are making two HST's at a time, i.e. drawing a diagonal line across your square then sewing a scant 1/4 inch on either side of the line. Finally, a cut is made on the drawn line. This method requires larger squares for the two seams. Technically 7/8 inch, (1/2 inch for the two seam allowances plus 3/8 inch for the diagonal seam), but I prefer one inch over the finished size, so I have a smidge to trim for squaring up.
Another method would be to draw the diagonal line then sew on that line. This will produce one HST that is the correct size. One half of the square is your HST. The other half has the seam allowance, and the rest is cut away and discarded. It's not a preferred method because there is so much waste, you'd need a new set of squares for every HST, but if you've already cut everything one size, maybe it's an option to consider.