Originally, the information is presented graphically in a pie chart, but here for simplicity, the part in question is verbalized.
Travel sector 36%
Clothing sector 24%
Travel sector 29%
Clothing sector 16%
The description sentences as follow:
In 2003, the online sales in the travel sector accounted for 36% of total retail sales, which was 12% higher than sales in the clothing sector. The online sales in the travel sector and in the clothing sector declined by the year 2013, with 29% and 16% respectively of total sales by sector.
My question is about the appropriateness of the preposition of "by." The given information is about only two years, and whether there was a rise or not between them is unknown. It seems to me that "declined by" means a continuous drop without any rise. Please enlighten me.