I would suspect that SE is prioritizing packet traffic as it comes in to and out of their NA/EU Data Center out of desperation to remedy their capacity issues. (This is called "Traffic Shaping".) If you look at the Ping rates of all the NA/EU Servers you'll find a 100+ ms difference between the lower populated servers when compared to the higher populated servers. When servers are located in the same physical location on the same physical subnet they should have comparable Ping rates. Otherwise, if there would be a difference, the higher populated servers which are receiving more traffic should be the ones to have higher Ping rates.

For the fact that the higher populated servers have significantly lower Ping rates across the board shows that SE recently made some routing changes at their NA/EU Data Center to help alleviate congestion on the higher population servers, but at the cost of the lower population servers experiencing an increase in lag.

As a band-aid they are probably doing a combination of Packet Shaping, Congestion Control, and Traffic Control to deal with the Bandwidth Contention (oversubscription, or too many connections sharing the same limited network bandwidth) at their NA/EU Data Centers. It will probably remain this way until their infrastructure is upgraded in the coming days (or weeks).