Unless you have done actual experiments and measurements and had breaker trips, I suspect the situation is a little different.
The "standard" continuous use device for a US/Canada residential standard 15A circuit is 1500W. This is why nearly all portable electric space heaters sold in the US/Canada are 1500W. It doesn't matter whether they say "big room" or "small room" or specify an area. It doesn't matter whether they have a fan or not, ceramic, oil filled, radiant, etc. 1500W is the magic number.
Similarly, the "standard" short term use device for a US/Canada residential standard 15A circuit is 1875W. This is why nearly every hair dryer (whether $15 or $100) sold at retail for the US/Canada is rated 1875W.
Because the baseline voltage is actually 125V, not 110V. (It was 110V and then 120V and then 125V, though the typical actual voltage in most areas is still 120V.) 125V x 15A = 1875W - that's your short term usage (a.k.a., hair dryer). Continuous usage requires an 80% derate, and 125V x 15A x 0.8 = 1500W - that's your space heaters.
Which means that while you shouldn't actually use more than 1500W on a continuous basis on a 15A circuit, you won't actually trip any breakers until some point past 1875W. You have 3A of wiggle room. In reality, you can typically go 10% to 20% over the rating for an extended period of time without a likely breaker trip. But you shouldn't plan that way.
Depending on your situation, it may be possible to run 12 AWG wire and use a 20A breaker. In fact, the NEC (electrical code for the US) has required 20A circuits for kitchen (toasters, coffee makers) and bathroom (hair dryers) for many years, even though in residential usage most 20A circuits legitimately have 15A receptacles (this specific exception to breaker size = receptacle size is part of the NEC). That allows for a full 15A usage (which is either a true 15A hair dryer or a derated 12A space heater) to have 5A (or 3A continuous) of room to spare. Meaning that if you have a 15A circuit with a 1500W space heater you really shouldn't have much else running on the circuit at the same time, but if you have a 20A circuit then running lights and TV, etc. on the same circuit won't be a problem at all.
A similar calculation should apply to your circuit, unless you are using a specialized very sensitive/accurate circuit breaker. But with standard items (typical residential/commercial breaker panels from Eaton, GE, Siemens, etc.) this is how it works.
There is also a little extra room provided by Ohm's Law. A typical space heater, not counting fan or other extras, is a simple resistive circuit. For 12A @ 125V = 1500W, that is 10.42 Ohms. Which means that at 120V (typical for US/Canada) the actual usage will be 11.52A and 1382W. (Well, some slight variance from that is possible due to resistance varying slightly with temperature and other factors.) So in a typical setup you actually have a little extra current available in the circuit for other stuff before you run into circuit breaker trips.
For more about how circuit breakers work and why a 15A breaker doesn't magically trip at 16A, search for trip curve.