Anaerobic difference between 1030 Plus (bike) and Forerunner 955

There can be multiple reasons. Like is your HR settings in line in both units.

Which do I adjust? Why would the 1030 Plus have anaerobic and the Forerunner 955 have only high aerobic? I've gone through the HR settings of both, they allow you to control them differently, they don't have the same UX options. How do I sync them?

I don't know how one should be doing that.

As if you have different HR settings, then do activities in both, there is going to be difference. You should just log the thing with one device.

Also like if your 1053 is having HR settings as XYZ, and then 955 has some autodetections and they change don't know how Garmin is going to sync these.

So I don't have any solution to give you how one should keep the both devices HR settings aligned.

So you think my max HR is different in them? Which setting should I be adjusting in the other?

I think you have different HR settings, different HR Zones.

https://www8.garmin.com/manuals/webhelp/edge1030/EN-US/GUID-94A5A126-6BB2-47F6-8040-EAD29EC66C2B.html 
https://www8.garmin.com/manuals/webhelp/GUID-9D99A9D4-467A-4F1A-A0EA-023184FEA3DD/EN-US/GUID-30C91919-943C-44E9-8048-901AC0881AEA.html
So would look at those and make them match.