Because these are two different products.
The Swiss Half Fare Card is destined for tourists, is valid for 1 month, and costs CHF 120. It can be bought from the Swiss Travel Center or from SBB.
Swiss Half Fare Card | Switzerland Travel Centre
Swiss Half Fare Card – Switzerland at half the price | SBB
The Half Fare Card is destined for residents, is valid for 1 year, costs CHF 185 (or 165 after the first year), is a subscription and therefore renews automatically.
The Half Fare Travelcard on the SwissPass | SBB
→ The prices for the two half fare cards are the same, no matter if bought online or offline.