When a HubSpot portal is integrated with Salesforce, the HubSpot Owner contact property is populated with a Salesforce's Lead/Contact Owner value. However, in some instances, when reviewing the HubSpot 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, whose owned by a missing owner, syncs over to HubSpot, because this will make that missing owner value show up in the HubSpot Owner contact property. As an alternative, you can add the Salesforcer 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 him/her under the HubSpot Owner contact property.