Is the article an correctly used here

Here is the first sentence from an economic article:

"According to STATS’s preliminary estimates, broadly in line with
expectations, GDP has fallen by 12.1% YoY in February 2021, following an
11% YoY drop in January 2021."

Is article "an" in bold face in the above sentence correct? Should not it be "the" instead? since the 11% is specific not general. It is that 11% of january which we know.

If "an" is correct then what is the rational for it?