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What is the configuration?

5v USB port < USB meter < power bank

The power bank was 25% full based on its indicator.
Was the USB meter’s capacity reset to Zero when you plugged it in (capacity reading needs to be reset to zero first if measuring capacity)?
Assuming the USB meter’s capacity display was reset to zero, and the capacity charged reads 2000mAh, and the powerbank became full (display reads 100%), right?

Then that means 75% capacity is 2000mAh, so total capacity of the powerbank would calculate to something like 2667mAh.

However, my guess is the capacity readout (25%) of the powerbank is a “guess” [not that accurate] based on the voltage of the lithium-ion battery inside the powerbank, and not the actual capacity. Another thing is the power bank may have degraded in capacity (if this is not a “new” powerbank).

I recall testing a 10000mAh powerbank when it was relatively new (I didn’t have a USB meter yet when it was brand-new), but when I got that powerbank it had several cycles of usage, and I measured something like 8000+ mAh capacity. But right now (after 2 or so years), when I do another capacity test (it now takes a very longer time to “fill up” the last capacity, and it’s just something like 3000+ mAh capacity (from empty, charge to full, measuring capacity with the USB meter in between). Seems like a big degradation…

Yup, USB 5v vs lithium-ion nominally 3.7v so there would also be an efficiency factored in the equation…