Assume you have traded back in 2016, the tax report needs to calculate the 2016 trades as well to get the right remaining assets and the correct cost basis.
The profit from 2016 will of course not be added to your 2017 report, but it's important to get the remaining assets on 12/31/2016 for a proper start of 2017.
If you set a filter in the gains report, all previous trades will not be included.
And that leads to different values.
You can recreate this by going to the gains page and setting the "sales date end" filter to "12/31/2017"
Now the report will show all your realized gains until the end of 2017.
Now go to the tax report and create one report for 2016 and another one for 2017
The sum for both years will be the same as in the filtered realized gains.
Because of the fact, that the gains page is only calculating in the date range you set in the filter, and the tax report calculates your previous trades as well, the results are different, but both correct in their own way.