Baby sizes!

What is up with different clothing companies not sizing baby clothing different sizes?? My daughter is currently in 0-3 months clothing and some things are completely different measurements! I was going through some of her clothes that we bought and put away for when she's bigger and found a crown sleeper sized 0-3 yet it's the same size as one of her 12 months sleepers! The cupcake sleeper is her current 0-3! I know it could just be a printing accident but it just annoys me that baby companies don't just do the same size measurements!