Is the use of the term "capacity" in the following sentence right?

```
ICU capacity dropped below the 15 percent threshold in some regions.

```

I has been a bit baffled. The word "capacity" means the maximum amount that can be contained, so I though it a constant fixed figure that does not change easily--at least in the short term unless new ICUs are built.

But the sentence above, from an article on a major news site, says it "dropped", implying a change--a decrease--in capacity (supply of ICUs), as patient numbers (demand for ICUs) increased.

Please tell me how to define and understand the word "capacity."
Thanks for your exceptional knowledge.