I think the "issue" starts with the size of the cut pieces.
For example: even though I think I cut "exactly" - when I measure my 2-1/2 inch strip - it is actually about 2-9/32 inches wide - about a rotary blade's thickness wider than the template.
So I can use a "regular" 1/4 inch seam - and when sewing non-bias edges together - the unit usually ends up being the size I am aiming at. There is enough "extra" in the way I cut to allow for the thread thickness and the amount of fabric taken in the turn/fold.
If my cut pieces were EXACTLY 2-1/2 inches, then if I used an EXACT 1/4 inch seam, there would be some "loss" when I pressed the unit - the fabric has to turn and accommodate the thread - and the result would be less than 4.5 inches - as Dunster said.
I think the place to start is to measure the size of one's cut pieces. A couple of threads here and there can make a difference in the finished size of the unit.