The robbers' dynamite is detonated when it is shot. Dynamite is too stable to be set off by a bullet impact.
The outlaws steal dynamite from Ramirez's supply wagon. Dynamite was invented by Alfred Nobel in 1866. 33 years later.
Ramirez comes into town asking about a pair of robbers who stole a shipment of dynamite. The robbers killed the wagon drivers so there would be no witnesses to report back to Ramirez, not even on how many of them there were.