Some groups, depending on the plan chosen and the date the group was formed, have more features and amount of storage. All groups have comprehensive moderation tools and message archives. 

Check the dates that some basic (free) group functions were changed and were legacy enabled (grandfathered).

https://groups.io/g/GroupManagersForum/wiki/25069

Changes from February 8, 2021:

For groups upgraded after Tuesday, February 8, 2021 at 9am Pacific Time (17:00 UTC), the pricing plans will be based on the number of members in the group. The base costs of the different existing plans are not changing. The change is that above a certain number of members, the per member pricing kicks in. 

For details, see https://beta.groups.io/g/main/message/27191
Delayed to the 8th: https://beta.groups.io/g/main/message/28260