Ok I did a quick research and found that it’s a thing with ViaBTC mining pool in general.

And ChatGPT was telling me this:

“ViaBTC blocks are often not full because the pool prioritizes building block templates quickly (to reduce the risk of stale/orphaned blocks) and uses a transaction selection policy that includes their “accelerator” service. This means they don’t always optimize for the maximum fee revenue per block.”

In case you were interested too ;)