[j-nsp] confused about "Not Best in its group"

[Min]

Sometime BGP inactive route shows inactive reason as "Not Best in its
group". However, if configured "protocols bgp path-selection
always-compare-med", inactive reason will be more specific, like "IGP
metric", "Interior > Exterior > Exterior via Interior". Really
confused how "always-compare-med" would cause this ( specific inactive
reason rather than "Not Best in its group" ) while best path selection
no change. Can someone explain this? Thanks.