It all comes down to availability.
It seems like just yesterday Ford was celebrating the Mustang dethroning the Camaro as America's top-selling muscle car. Now we get word that GM has sold more trucks through the first half of 2015 than Ford. Ostensibly the numbers look pretty bad: Chevy Silverado sales have jumped 14.6 percent this year while F-Series sales dropped 2.4 percent. However, if you've been paying attention to Ford and the F-150 then you knew this sort of drop-off was inevitable.
Ford just started making the F-150 out of aluminum, a process that required a complete overhaul of its assembly line. That shut down production for 12 weeks, causing Ford to lose an estimated 90,000 trucks. That gave GM an opportunity to sell Silverados and Sierras to commercial buyers as well as folks looking for a brand spanking new truck. Now that Ford's plants are back up and running it'll be interesting to see how the numbers go. It'll also be interesting to see if GM does anything to try and hold its lead.