I think, first, we have to know what SPF means:

The SPF is based on how long your skin can stay in the sun without burning compared with unprotected skin. So someone whose skin starts to redden after 10 minutes on the beach would get 150 minutes of skin protection from an SPF 15 sunscreen and 300 minutes of protection from an SPF 30. The difference is the time, not the protection level. And bear in mind that the result is based on ideal lab tests. In reality, most people use much less than recommendation (some say only 1/4 was applied on average) and do NOT reapply as instructed.

Higher SPF sometimes give us wrong message that we can stay in the sun for the whole day, which is really dangerous.