As per[1]:

In a clustered table, BigQuery automatically sorts the data based on the values in the clustering columns and organizes them in optimally sized storage blocks. You can achieve more finely grained sorting by creating a table that is clustered and partitioned. A clustered table maintains the sort properties in the context of each operation that modifies it. As a result, BigQuery might not be able to accurately estimate the bytes processed by the query or the query costs.

However, in some cases the estimated bytes shown is the size of the full table scan and does not take into account the partition size.

Therefore, assuming this is provided as best effort basis, there still might be some room for improvement on Bigquery dry_run calculations.

[1] https://cloud.google.com/bigquery/docs/clustered-tables#clustering_partitioned_tables