My thought is that the price you pay is a “unit price” for a bunch of celery and a “net weight” price for the sticks.
The sticks are packaged at 300g, and the machinery/process ensures that they add enough sticks to weigh more than 300g, plus some fluff to cover water loss as the product ages. Going to 450g seems excessively generous, but there you go.
Bunches are not packaged by weight, they’re packaged by unit, but they need to put a weight on the package for regulatory purposes. So, what weight to put on the package that will suffice for every single bunch of trimmed celery that could possibly wind up in it? Something far lower than what you would expect to put in there, but not so low as to confuse your customers.