The original recall plus the 403,000 additional units cover a number of Graco seats for older children that face the safety campaign because they have buckles that may not release. According to reports, some parents were forced to cut their kids out of the faulty seats. According to a NHTSA documents obtained by The Detroit News, the additional units include the 2006 through 2014 Argos 70 Elite, Ready Ride, Step 2, My Ride 65 with Safety Surround, My Size 70, Head Wise 70 with Safety Surround, Nautilus 3-in-1, Nautilus Plus, and Smart Seat with Safety Surround.
As for why NHTSA wants the infant seats recalled, it's a slightly more difficult issue. Yes, as NHTSA says, the buckles are the same. Graco argues, however, that the infant seats are used differently, according to The News. The manufacturer also argues that in the event of an emergency, parents can simply remove the entire seat from the car - the same can't easily be done with the toddler and booster seats.
According to Kicking Tires, the primary issue with the buckles is that food and drink could gum up the mechanism, preventing it from opening. Owners of affected seats will be sent repair kits with instructions on how to safely clean the buckles, although Graco claims the seats are safe to use in the meantime.