At the end of the routing process, the router reports an estimated timing summary calculated using actual routing delays. However, to improve run time, the router uses incremental timing updates rather than doing the full timing computation to calculate the timing summary. Consequently, the estimated WNS can be more pessimistic (by a few ps) than actual timing. It is therefore possible for the router WNS to be negative while the actual WNS is positive. If the router reports estimated WNS that is negative, the message is a warning, not a critical warning.
route_design -directive Explore, the router timing summary is based on signoff timing.