I don't know how they are accounting for the various different data centers within a region (IE. Availability zones). It could explain self latency if a packet is leaving a DC
A single availability zone is often (always?) composed of multiple datacenters which are spread relatively far apart from one another.
does multiple buildings at single site count as multiple DC? or it must be multiple physically separate locations?
yes, while in my opinion most services won't care about the distinction between cross-az/same-az (you're definitely not most services :) ), you can definitely tell the difference between
* in the same region vs not
* in the same az vs not
* in the same placement group or not
which shouldn't be surprising. you should get a latency benefit from increased physical proximity!