Machines are notorious for significantly overestimating calories. However, HRM's are not 100% reliable either - high blood pressure, some medications, etc can cause issues. And given they are based on broad population averages, statistically they are a good predictor for 1/3 of the population, somewhat good predictor for another 1/3, and less useful for the remaining 1/3.
One approach might be to check your HRM walking or running (these are well studied and understood activities, and online estimates are reasonably reliable).
To put things in context, even if the machine were 25% off, and you were burning 300 calories rather than 400, when combined with say, knocking off 500 calories through watching your intake, this gives a calorie deficit of 800 rather than 900 - the impact of the error is not all that significant. (Mis)estimates of calories burned should not prevent you from losing weight.
Not that I have anything against trying to get a more reliable estimate.