Understanding storage allocations in SharePoint Online can be a bit confusing. If we look at the storage allocations for the E and P plans....
10 GB base customer storage plus 500 MB per enterprise user
10 GB base customer storage plus 500 MB per user
Storage per Kiosk Worker
Storage per external user
Available at a cost per gigabyte (GB) per month. See this blog post for more.
Site collection storage quotas
Up to 100 gigabytes (GB) per site collection
My Site storage allocation(individual) (1)
500 MB of personal storage per My Site (once provisioned) (2)
Total storage per tenant
Up to 25 TB per tenant
(1) does not count against tenant‘s overall storage pool(2) the storage amount on individual’s My Site storage cannot be adjusted
...we see that there are two kinds of storage allocations - pooled and individual.
Pooled Storage - As seen in the above table each customer tenant in SPO receives a default amount of 10GB of storage. Users under K plans and external users do not contribute to the pooled storage. Currently, pooled storage has a 25TB limit in the E plan (35GB for the P1 plan)
Individual Storage - End users who get a My Site (E1-4 SKUs – P1 SKU doesn’t include My Site) receive 500MB of personal (individual) storage when they first self-provision their MySite (when they click the "Content" tab of their My Site). This storage is in addition to the pooled storage allocated to the customer tenant per USL, but is not aggregated.