Because it does not sit at The MLC forever. Starting in early 2027 they begin distributing accrued royalties that were never matched or claimed, by market share, splitting them among the publishers and writers already collecting in proportion to what they already earn.
It goes out oldest usage month first, and once a month has been distributed it is gone. Claiming and matching stay open indefinitely, so anything you fix now protects every month not yet paid out, but each month that passes with your name missing from a recording is a month of its royalties handed to somebody else. The MLC's own explanation ↗
How we size it. We take your Spotify stream count, estimate what that means across all services, take the US portion, and multiply by the US mechanical rate. Then we keep only the part that is actually exposed. Usually that is the unclaimed percentage of the work. Where the work is fully claimed but the MLC never matched Spotify's own copy of the recording to it, the money behind those plays has nowhere to go either, so none of it is discounted.
The per-stream rate is measured from MLC distributions and historical CRB Phonorecords rates. We checked it against catalogs we administer, where the rate implied by actual payments came out around half the headline figure, so we use the measured one instead.
The MLC also publishes its own ranking of what each unmatched recording is worth, and we read that too. Where their figure is larger than ours, we use theirs. We never add the two together, because they are two estimates of the same money rather than two piles of it. This mostly shows up on quieter recordings, where a handful of streams tells us very little and The MLC is holding real money anyway, and on recordings we have no stream count for at all.
It is a range rather than a single number, give or take 35%, because every other input is an industry average rather than something we measured for you. A trailing plus sign means the top of the range is open-ended: The MLC's highest band has no ceiling, so we cannot name one either. Treat it as an order of magnitude, not an invoice.