When a HubSpot portal is integrated with Salesforce, the Contact owner contact property is populated with a Salesforce's Lead/Contact Owner value. However, in some instances, when reviewing the Contact owner contact property in HubSpot, you may not see all of your Salesforce owners. This is because the missing owners have not had a Salesforce lead/contact synced to HubSpot.
The solution in this case is to simply wait until a Salesforce lead/contact, who is owned by a missing owner, syncs over to HubSpot, because this will make that missing owner value show up in the Contact owner contact property. As an alternative, you can add the Salesforce owner in question as an actual user in your HubSpot portal (with the same email address used in his/her Salesforce profile). Doing so will add them as an option under the Contact owner contact property.