They are two different limits. Open spaces are about whether a two-pole breaker can be physically installed. Service capacity is about whether the calculated or measured load, plus the new charger, stays within the main-breaker or service rating. A panel can look empty and still be electrically full, or look crowded and still have amps to spare.
This is the distinction that made the 100-amp / 60-amp Reddit thread useful: several electricians said you could rearrange breakers to make a two-pole space, and then said that was the wrong question. The right question was the load calculation, and the 100-amp main breaker — not the size of the can — was the service limit.
Related traps: a 200-amp-rated enclosure with a 100-amp main (see that page), tandem breakers that fill every slot of a 100-amp panel, and a full interior with plenty of unused capacity on a 200-amp service. The calculator only addresses capacity. A licensed electrician still has to look at the physical panel, its listing, and its condition.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.