When you cast iron plates, they'll be some overage or underage on the stated weight. On the cheapest plates there is no guarantee on it matching the stated weight, so you can see wild swings on the plates. Better gym quality plates are usually guaranteed to be -/+ 2% of stated weight, and competition plates are guaranteed to within a few grams.
You can negate having to use competition plates by weighing them on a calibrated scale, but then you'd have to check the scale, and trust the meet director, etc.... Most of the larger feds now call for the plates to be competition for it to count.