Their site's fare calendar is a dark pattern. Normally when searching for round trip fares on, say, Google Flights, selecting the departure date from the calendar shows the (lowest) round trip fare departing that date. When you then select the return date, the (lowest) round trip fare departing on your departure date and returning on your return date is shown.

Cabin's site shows you the one-way price both times even when booking a round-trip. It's not until you click through that you see it's $115 each way.