I understand here "bias" to be the uncertainty of measurements. So the order will be the following:
6.4 ± 0.5 s
6.6 ± 0.1 s,
6.63 ± 0.01 s,
6.52 ± 0.05 s,
(notice how the second number, the one behind the symbol ± gets smaller, as the bias gets smaller).