The saving for booking future cruises comes from buying CruiseNext vouchers onboard. This effectively gives you a discount (the actual amount depends on how many you buy and how many you can apply to your booking, which varies depending on cabin type and any special offers).
You don’t need to use these onboard, just buy the vouchers. They can be applied when you do book.
The main advantage of actually booking onboard is that they can usually give you a small upgrade. This is just to the next cabin subcategory, so the saving is not huge, but should save you a bit.
Just ignore the sale prices. They aren’t starting to become like furniture store sales, they have been well established as such for years. The sale changes but the price remains the same.