tag removed – Adding Sq Ft To Area Already Given

I measure an apt 23ft x 15ft which = 345sq ft. Now I want to add 2 feet to each side, and I get 349 sq ft. Yet, if I go 25ft x 17ft, I get 425. I’m not clear on why it doesn’t work the same way. If I add 2 feet to 23ft, it’s 25 ft; and I add 2 feet to 15 ft, and it’s 17. It’s still adding 2 sq ft! Yet, it’s not the same thing.