Yes and that is an average assumption of standards, and what these are remains a mystery, non of the heaters explain how they come to them. As glass thickness, flow, open top or closed and ambient temperature are all factors having influence on the heaters performance.. Next to what stands 100 watt for, obviously in the first place power consumption and not realy accurate capacities. As you experienced yourself a 100 watt hydor seems much more sufficient than a 100 watt eheim. And both consume the same amount of energy well in operation.
What maybe could help is placing the heater in another spot where flow is optimal.. Could be the heaters thermostat switches off while the other end of the tank still isn't at the set temperature. It likely is the build in bimetal thermostat (which are known to be very inaccurate) playing your parts, using a external electronic digital thermostat with the heater set 2 degrees higher than desired temp and do the rest external could also result in a huge difference. The inline hydor also uses an digital thermostat.
Digital thermostats can also be calibrated to the thermometer in use..
Another thing to keep in mind is the thermometer.. I have such a hangon liquid filled thermometer sold by one of the sponsors, which says 18°C while another digital thermometer says 20.9°C and the digital TDS scanners thermometer says 22°C.. That's quite a discrepanty all stating beeing very accurate in the manuals description. 😵 Seems each manufacturer has its own concept of accuracy. In reality with the 4° difference i measure with 3 different meters i do not know what temps my tanks are other than warm enough since i have no issues. 🙂
