Generally speaking, most mower companies don't have model years, they have model "runs."
The HRX217 has been around almost 10 years, and has been updated along the way. Honda refers to these running changes as "K-change." Think of them as software versions.
The very first HRX was a "K0" and over the years have seen the K2, K3, and last January, the K4 version was introduced along with some new models (all were set to be K4 for consistently) . Same basic mower, but different features for each K-change. Honda has been pretty aggressive in updating mowers, but it comes and goes in cycles. All the HRR and HRX models were updated within the last year, but the HRS and HRC versions have been unchanged for many years now. Eventually, they will either be discontinued or updated.
Under most market conditions, retailers will sell out of a K3 before K4 models are released. Sometimes, there is overlap, (and customer confusion). We saw this this spring with many K8 versions of the HRR models were still on the shelves but the K9 versions were also arriving in stores. Dealers will often discount slightly older models in this case, but it does not happen very often.