We aim to have the highest level of precision in our numbers. If you manually take a total of revenue from 1 day (Which contains many items), and you take the percentage VAT of this amount, it does not necessarily have to match exactly to the Mews reporting.
Consider item that costs 0.01 EUR. For such item, VAT is 0. and net is 0.01 (because it's rounded to 2 decimal places, if the precision was higher, it would be something like 0.001 VAT and 0.009 net). And consider 100 bills with one such item on each. On each bill, you must have a tax summary and since the bill contains just one item for 0.01 EUR, the VAT on the bill is 0. And now if you take the report, there is sum with value 1 EUR (100 times 0.01) and VAT is 0 (100 times 0). This way it matches the tax summaries on the bills, if you calculated VAT from the sum 1 EUR, it wouldn't match the tax summaries.
In addition to that, VAT count is always done from Net amount, so: in case your VAT is 6% and your total was x, your NET = x/1.06 and tax = Total - NET