"Half-life" by biological elimination is not precisely exponential as with radioactive decay, and may trend closer to a linear process -- as in, your liver can eliminate X units per hour, not X% of however much you've had.

Thus the (informal) "half life" of one cup of coffee could be 4 hours, but two cups could be 8 hours, and so on.

(To the extent I can track the estimates of "3.5 to 6" or "5.7" hours of caffeine half-life back to original sources, it looks like the studies were done in 1978 and 1980. Maybe modern caffeine consumption has moved to a level where linear rather than exponential elimination dominates?)