Just to clarify, the Contacts app on your tablet and the Contacts app on your phone are not syncing with each other, they're both syncing to the Contacts list in your online Google account. It's your Google account that's the common focal point that both your phone and your tablet simply work off of.
At some point, since it sounds like you have contact entries that may or may not exist in both your phone and tablet, you need to just sit down, go through ALL your contact entries, and manually add/combine/edit your main Contacts list using a web browser and going to;
https://contacts.google.com
-- Turn off syncing for Contacts in the Settings menu of both your phone and your tablet
-- Go through the process of checking what contact entries are on your phone and compare that to what's in your primary contacts list in your Google account, and manually add or edit it accordingly. If you have a lot of contact entries this is a pain but you need to get things straightened out to have syncing work automatically.
-- Do the same with your tablet
-- Once you get your Contacts list all straitened out in your Google account, that should contain
all your contact entries, not just some on one device or the other. You might even want to use the Export option to save your Contacts list as a csv file as a backup.
-- Now on your phone go into the Settings >> Apps menu, find and open the Contacts app, tap on Force stop, then find and tap on the Clear data button. That should reset the Contacts app back to its original, first-time used state. Now go to the Google >> Accounts >> Google menu and enable syncing with Contacts again. Start up the Contacts app and if it doesn't automatically start loading up your Google account Contacts, check the app's Settings so the app is tied to your Google account.
-- Go through the same process with your tablet.
