There are a lot of factors that affect the number of miles that you get on a charge. Temperature, auxiliary electrical loads (like heat or AC), terrain, driving style, and driving speed. The indicted EV range is an estimate based on a short history of past drives. The EV range gauge is sometimes referred to as the Guess-o-Meter (GOM) on this forum. You can search this forum and find MUCH discussion on EV range. I don't know what you mean by "39/36"... Is that the estimated range from two different charge cycles? It is possible that the vehicle is still learning your driving pattern and will make some adjustment to the GOM reading.
If you aren't using heat or AC, and you drive with a light foot (economy mode) without triggering the ICE to come on, and you drive on mostly flat terrain, and you drive at mixed speeds below 60 mph, then 36-39 does seem a little low.
You mentioned that you have not been using heat or AC. What about the nature of your drives? Freeway driving will yield substantially less range than city or mixed (particularly at speeds > 55 mph).
Ultimately, it is the remaining capacity of the HV battery that defines the range for a given scenario. It is also this capacity that Honda uses when determining the warranty status. There is no way to obtain the battery capacity except with professional equipment or at the Honda dealer. The nominal capacity of a new Clarity is 54 Amp-Hours. Battery capacity is expected to deteriorate some over time, but a 2018 should not have dropped very much. Honda's warranty applies when the capacity drops below 66%.
You can get a feel for your capacity if you have an EVSE that records the energy input during a full charge (in kilowatt hours). A full charge when new is something like 14.5 kWh from the EVSE. My 2018 now takes around 13.5 kWh (down 7%) but I reliably get more than 50 miles of EV range with my driving scenario.
If in doubt you can have the dealer check your HV battery capacity. Many dealers are inept with this vehicle, so we can guide you as to how to specifically ask for this measurement. If you just ask them to check your battery they WILL do the wrong thing and measure your 12V battery giving you a useless result.