When you send your product to FBA, Amazon will verify the weight and dimensions of the product to calculate the Pick and Pack Fees. Amazon does this each time you send a brand new product, however can also re-measure and re-weigh existing products during receiving and change the dimensions or weight without informing the seller.
These measurements at Amazon FBA are meant to be accurate, but mistakes do happen. And, it is up to the seller to catch the mistakes, otherwise it could cause you to lose in some cases a significant amount of money if your product is pushed into a higher fee price tier.
One of our clients received a credit $67,721.43. This seller sold 200 units per day and the dimensions and weight were off by an inch. Luckily a case was opened and the money was refunded. This money that belongs you.