Seam allowance is only one factor in getting a block to end up being the expected/hoped for size.
Was the fabric washed/shrunk before cutting? If starched/sized - the moisture and heat when it's pressed can change the size of SOME fabrics. Steam pressing can also shrink SOME unwashed fabrics.
Size of the pieces - ruler placement can make them vary up to 1/16 or an inch or more - Quilter's Rule has really thick lines - so I don't use it now.
Even using the fine lines on the rulers I use now as guides, I've noticed that my 2.5 (2-1/2) inch strips are actually about 2.52 (2-17/33) inches wide. If you use the shape-cut ruler or die cut pieces, they might be a bit smaller.
Thread thickness - some think this really matters -
Pressing techniques - try not to stretch the fabric - but make sure that there are no pleats by the seams
Seam width matters - but what really matters is that the unit or block ends up being the expected/wanted size.