CREATE/REFRESH MATERIALIZED VIEW planner difference?

Hi all,
Should I expect a planner difference between CREATE MATERIALIZED VIEW and REFRESH MATERIALIZED VIEW? We have a
materializedview that uses 4 workers during CREATE but only one worker during REFRESH, and as a result the refresh
takesmuch longer (~90 minutes vs. 30 minutes for the CREATE). So far this behavior has been 100% consistent. 

I'm running both the CREATE and REFRESH on the same server (Postgres 11.9 on AWS Aurora). I don't think the refresh is
usingone worker in response to other things happening on the server because we’ve observed this happening when the
serveris not busy. We're not using the CONCURRENTLY option for REFRESH. 

THanks
Philip