Under-counting own losses
Militaries routinely under-report own casualties during active operations. Sometimes the under-reporting is deliberate; sometimes it reflects a slow chain of confirmation. Independent monitoring (insurance industry data, hospital admissions, leaked records) tends to surface true figures over weeks.
Over-counting other side's losses
Round numbers in early reporting ("100 killed", "500 wounded") are usually estimates extrapolated upward. Subsequent revisions often move the figure down.
Methodology arbitrage
Different counters use different methodologies. Hospital intake versus on-scene counts versus ministry-confirmed identities can each produce different totals for the same incident. Summing them is wrong even when each is internally consistent.