Judges 16:13 (and also verses Judges 16:14 and Judges 16:17) give us the only physical description of Samson in the narrative: his hair was braided in seven locks. If Samson had a physique like Superman, then the Philistines would have assumed that his strength was exclusively muscular. But they do not know the source of his strength, which suggests that he was an ordinary-looking man, other than his hair length and style.
It is quite possible, therefore, that the Philistines believe that Samson’s power was either supernatural or magical. This would explain why they believe all his (misleading) suggestions for removing his strength (Judges 16:7, Judges 16:11, Judges 16:13, Judges 16:17).1
13 Then Delilah said to Samson, “Until now you have mocked me and told me lies. Tell me how you might be bound.” And he said to her, “If you weave the seven locks of my head with the web and fasten it tight with the pin, then I shall become weak and be like any other man.”