Akshually, I think I have a better definition.
The metric shows how close a mined block came to including the “ideal set of transactions” in terms of “miner incentive,” i.e. the theoretical most lucrative lineup of transactions based on fees. And we all know that most spammy txs get a break from the witness discount, which makes their data cheaper, thus allowing them to bid effectively against normal monetary txs.
So if a miner deliberately leaves out spammy high fee-rate transactions, the block ends up with a lower health score, because from the metric’s perspective they left money on the table.
That’s an idiotic way to describe a block’s “health” imo. It’s really more like a miner’s paycheck “health”. A more intuitive measure might be the proportion of monetary transactions in a block versus all other types of datacarrier transactions: if all are monetary, it’s 100%; if it’s 50-50, it’s 50%; if zero, it’s 0%, and so on. Maybe the dudes at mempool.space might consider renaming it. Just my two sats.