My experience with cycling: calories burned are just slightly above the value reported by another application that bases its math on GPS data and averages for my demographic, but still in that ballpark.
My experience with walking: calories burned are almost twice the value reported by the same "other app". Which one is right? Well, of course the Alta HR! 
But seriously: in theory a device that monitor your heart rate should be able to give you a better estimate of the calories burned.
That said, I use these devices/applications to get some guidance rather than expecting "The Truth (tm)". I mean, have you ever noticed that some people are sort of thin even though they eat a lot, and get little to no exercise, while other people seem to gain weight just for thinking about food?
My point: not two people are exactly the same, even if they may belong to the same demographic, and let's not forget that the feedback we receive is still based on "averages" -- but averages plus heart rate reading are more likely to get you a bit closer to "The Truth (tm)".
RT.
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The thing is that fitness trackers can't directly measure the calories being burned: to do that you would have to spend a very uncomfortable day in a lab, attached to medical equipment, being prodded on a regular basis, etc. As far as I know only professional athletes do that and not very often.
However, all this data is widely available so there are statistics telling us how many calories are burned on average according to sex, age, height and weight depending on different type of activities including none at all, that is, being at rest and the latter corresponds to the Basal Metabolic Rate (MBR) -- which explains why the Alta HR records calories even when you've not been wearing it.
Last but not least, although fitness trackers can't measured calories directly, they're equipped with motion sensors and, sometimes, with heart rate sensors (like the Alta HR) therefore, at least in principle, they should be able to fine tune the averages to each wearer and, hopefully, provide more accurate figures but, and that's the rub, let's not forget that these are no medical devices (it's written on the box too) and, as such, they've not undergone the necessary certification to be proven correct (and don't have to). In other words: take the data with a pinch of salt.
Concerning your 1700 calories being reported: at what point during the day? I guess that could be about right if it was late during the day and you haven't exercised much -- but keep in mind that even doing chores around the house adds up. What's the reading just before midnight and the counters reset? Remember the magic figure of 2000 calories/day and compare your value to that. Also: make sure that your entered your personal data correctly. Stating the obvious, I know...
Reece
Best AnswerWell, an educated guess is still a guess and, as if we needed any more proof that we can't expect these fitness trackers to be 100% correct, there's yet another research telling us that while they're pretty good at monitoring heart rate, they're a tad too generous when they have to keep track of calories burned – likely to keep us motivated. In summary, the article says:
“We were pleasantly surprised at how well the heart rate did – under many circumstances for most of the devices, they actually did really quite well. [...] At the same time we were unpleasantly surprised at how poor the calorie estimates were for the devices. [...] Errors on energy expenditure were far greater, ranging from the lowest at 27.4% for the FitBit Surge to the highest error of 92.6% for the PulseOn device.”
Should you feel so inclined, you can check the paper itself but, being a scientific paper, it's really rather dry – you've been warned!
Reece
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