Emails not sending from Other SMTP IP

Website 1: https://lifeready.com.au 

On one of my sites, I have setup Post SMTP to send via Proofpoint. I can see in the settings that; Postman will send mail via ?SMTP-STARTTLS to outbound-us1.ppe-hosted.com:587 using Password (Login) authentication.

When I do the test email for this, I can see the email comes from the IP 148.163.129.49 and is delivered without any issues. This passes SPF etc.

Website 2: https://www.hbfphysio.com.au 

However, on the second site it doesn’t seem to work the same. Looking at Post SMTP I can see that the settings show Postman will send mail via ?SMTP-STARTTLS to outbound-us1.ppe-hosted.com:587 using Password (Login) authentication. However, when doing the test email each email comes from a different IP within the range of 34.116.64.47, 35.244.81.13 and 34.151.99.100.

These all look to be Google mail servers? Why would Post SMTP not be sending via Proofpoint for this setup?

Both websites use SMTP Authenticated credentials from Proofpoint and have been setup the exact same.