Your comments are absolutely correct. However, in my case, the time of this observation, has been for 15 months and during the warmer season of 2020 the range was always in that range of 220 - 245. Everything else being equal I would expect a gradual decrease in range. I always use no AC in summer and only heated seats when it gets cold For some unknown reason it appears that my battery pack can now take on more not less charge capacity.
When you say you keep good records on the distance driven and the charging applied, are you tracking the kWh used that's displayed on the infotainment screen when you shut off the Bolt, and the battery percentage in the MyChevrolet app? Those 2 pieces of data will be a more accurate way to calculate your battery capacity than relying on GOM and energy added.
Specifically, you would charge to hilltop reserve (which resets the kWh used to 0), and note the exact battery state of charge % in the MyChevrolet app. Then you would drive for a while, but stay above 70 miles of range remaining, and then note the kWh used on the Bolt's infotainment screen and the battery state of charge % in the MyChevrolet app. Your battery capacity calculation would be:
kWh used / (max SOC% - min SOC%)
A 2017 would have about 57 kWh when new, probably closer to low-mid 50 kWh now. If your battery was truly gaining 30% capacity (300 miles vs 230 miles), you would calculate something in the mid 70 kWh.