2 Answers

Explained here >>> http://www.buzzle.com/articles/spf-sunscreen-meaning.html

Reader's Digest
May, 2014

Summer Skin: The Rules Have Changed
Use two coats of SPF 50, which allows only 2% of the sun's UVB rays to penetrate you. SPF 15 allows 7%.
Apply SPF every day, not just on those days you're at the beach. Regular activities like driving and other short exposures contribute significantly to sun damaged skin.
Your sunglasses should be wraparound, and you should wear a broad-brimmed hat. Sunglasses that don't fit properly or wrap around can let in 44% of the UV rays. At least get sunglasses with side shades. 
Sun exposure is risky at any time, not just those "peak hours" of 10 A.M. to 2 P.M.  

With all this information, it only makes sense for popular lotions to start including the SPF.  That way, they keep their market and clientele.  It's just good business.