some ideas about aerodynamics

I have read a paragraph about aerodynamics from "howstuffworks.com", 

"Let's assume that two nearby particles split up at the leading edge, and then come back together at the trailing edge of the wing. Since the particle traveling over the top goes a longer distance in the same amount of time, it must be traveling faster."

 

 

But I have a question:why the two particles will certainly meet together after spliting up?