Percentages always mean percentage **of something**.
In this case, let's say 16.7% of apples were green last year, and this year it's 19.0%. I'm sure your university course focuses *heavily* on apples.
That's an increase of 2.3% **of the total apples**.
But the **proportion** of apples increased from 16.7%, by 2.3/16.7 = 13.8%. The 13.8% refers to the **proportion of green apples in last year's harvest**.
General moral—to repeat—percentages are always percentages **of something**, and you have to be clear what that something is.