You have not saved all your contacts to your Google account. Those not in Google of course do not sync.
On many phones contacts can be stored in 3 places.
1. As Google contacts. These are stored on the phone and synced to your Google account (unless sync is turned off).
2. As Phone contacts. These are stored locally on the phone only and not synced.
3. As SIM contacts. Unlike 1+2 above, only names and one number per name can be stored. No addresses, email info, photos, etc.
You have saved some contacts as Google (aka Gmail) contacts, and some saved in phone storage or to the SIM card. You could have some in two places, or even in all three.
You really should get all your contacts in one place, your Google account . To get everything there, export all the non-Google contacts as a .vcf file. Import the .vcf files to your Gmail contacts, either from the phone or transfer the files to a PC and import from there.
Your user manual will cover just how to do all this in more detail.
Android since v1.0. Linux user since 2001.