No one knows. This entire thing is a temper tantrum.
He's apparently applying these tariffs under a different law than the one they used previously, which does explicitly allow the president to apply temporary (150 days) tariffs of up to 15%. So you would assume this is meant to replace the tariffs the SC struck down.
But he's also stated that the previous tariffs are going to stay in effect, just under a different legal theory. So in his mind, it's possible that he thinks this 15% is on top of the existing amounts, and he's literally just tariffing the whole world harder because he's mad about a decision by his judiciary.
Then again, maybe he's just a dementia-ridden old idiot who can't keep his own bullshit straight for more than 30 seconds.