I was under the impression that the snapshots were stored on S3 and so the original instance's storage type wouldn't be relevant. So why is the storage type listed under snapshots? Is the cost of the snapshot higher for IO1 higher than SSD?
There are a number of attributes returned by the DescribeDBSnapshots API (which the console is using) that actually describe attributes of the DB instance when the snapshot was made, which appears to include this one, the storage type.
For another example, note the display of an availability zone, here. Snapshots are stored, replicated, across multiple AZs, so that isn't an attribute of the snapshot, either.
There isn't a documented price difference.