Payroll mistakes are ordinary — a shift typed in wrong, a break deducted that you never took, overtime paid at flat rate. Most go unnoticed because almost nobody keeps their own count, and without one you are arguing from memory against a printed number. Ten seconds after each shift is all it takes to be the person with the record.
Only deduct breaks you actually took and were not paid for. If you worked through it, do not deduct it — that is exactly the sort of quiet unpaid time this app exists to make visible.
Your hours produce a gross figure. Tax, insurance and pension come off after that and vary. Comparing your hours to take-home pay will always show a gap that means nothing, so ShiftTally asks for the gross amount from your payslip.
A difference is not proof of anything on its own — rates change, bonuses and holiday pay land in odd places, and pay periods rarely line up neatly with weeks. Start by asking payroll, with your record in hand. This app does not give legal or employment advice and does not know the rules where you live; if something looks seriously wrong, take your printout to someone who does.