From the technical perspective, Verizon speed test records the highest "instantaneous" speed in a given period of time. Your GPON line might be saturated and the OLT is only able to transfer at gigabit speed for splits of seconds from time to time. A GPON line from the CO is shared among 32 or more customers. The total throughput of a GPON line is 2.5G/1.2G, if there are 3 gigabit users on a single GPON line, nobody will get gigabit constantly. I recommend you to ask a service representative to check the saturation of your GPON line. If your GPON line is indeed saturated, your ONT might need to be migrated to a separate GPON line or the line techs from your local CO would need to pull a new line.
If you never use a gigabit subscription to its full capacity, I recommend you to downgrade to 500 or 300Mbps. My household started at 100/100, then 940/880 was cheaper in 2017, then now 500/500 on Mix and Match.