General practice, not a claim about any one account.
DifferenceLocale, decimal and date formats
A sheet can read the same number or date differently when the locale changes. A decimal comma and a decimal point, or a day-first and a month-first date, can shift values and dates. Check the format of every import before you add anything up.
DifferencePasted values and hidden rows
Numbers copied by hand, pasted over old values, or hidden by a filter change the totals. A report built on a copy can drift from the source after the source is updated. Keep one raw tab and one report tab.
DifferenceCurrency and rounding in formulas
A sheet converts currency with the rate typed in, not the rate a platform used on the day. Formulas round at their own step. Totals built from rounded rows can differ from totals built from raw data.