[question]some mails don't be reflected on threads

Hi, all.

I faced a strange phenomenon.
One day I posted a mail to pgsql-hackers and the mail didn't be reflected on the thread.
The thread: 
   https://www.postgresql.org/message-id/flat/EDA4195584F5064680D8130B1CA91C45367328@G01JPEXMBYT04
The mail:
   https://www.postgresql.org/message-id/EDA4195584F5064680D8130B1CA91C453EBE79%40G01JPEXMBYT04

This occurred on the other person. e.g.,
https://www.postgresql.org/message-id/flat/71E660EB361DF14299875B198D4CE5423DE3FBA4@g01jpexmbkw25
https://www.postgresql.org/message-id/71E660EB361DF14299875B198D4CE5423DEBD26D%40g01jpexmbkw25

Here I have a question:
How are mails associated to each threads?
In my investigation and thought, it is the mail header "In-Reply-To".

The main purpose of this mail is to separate the cause.

Best regards,
---------------------
Ryohei Nagaura