If you look at wing planforms of e.g. the Spitfire, you can see that the sweep measured at the 25% chord line is 0°. However, the leading edge is rounded/slightly swept and therefore induces spanwise flow across the wing. The problem also applies to trapezoidal wing planforms where the leading edge is swept back and the trailing edge is swept forward, but the overall sweep angle is still 0°.
For a wing with 0° sweep, shouldn't the leading edge always be perpendicular to the flow direction? Why is sweep measured at the 25% chord line and not at the leading edge? Has that something to do with the aerodynamic center?