Good catch!
I've often wondered how much PBs try to "off-load" old inventory they have on jobs when they are signing up new customers. I have always assumed that PBs, like a lot of businesses, have to order some of their materials in bulk quantities that may or may not be used up. I've also wondered if PBs purchase equipment like pumps, filters, etc, in order to get the sales incentive bump back from the manufacturers without actually having a customer for the equipment lined up. Then, when they get a job, they try to offload what they have even if the customer is leaning towards some other product...I'm not trying to tar all PBs as bad-actors, but stuff like this happens all the time (and it's one of the reasons why a company like Amazon exists and can offer great deals) and I've got to imagine that the variability in the pool building world means that there is going to be some slack and bulge in the supply chain. Also, one can't expect that a manufacturer of certain types of products, like plaster and aggregates, will always have a stock of materials ready to sell. If you think your favorite supplier is gong to run short of equipment or materials mid-year, then there's a lot of pressure there to "stock up" in the hopes of booking work that will use it.
Just wondering...