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You're forgetting the offset from undercutting. The cars don't need to be within 1s on track for it to matter - you could be within 4s and that extra 0.5s in the pit stop costs you the position if you pit later. Un-lapped 'traffic' is also critical. If you're trying to find a gap to pit into and it's tight an extra second could put you behind a slow car and cost you 'real' positions later.