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Its because your car burns gasoline for powerf. Ok J/K..
Really the reason is because your catalytic converter and car are not up to temp yet. When you first start up the car your fuel pressure and RPM are raised slightly to help warm up the car quicker. This is also to help warm up the cat so it starts working as soon as possible for clean emissions. If you notice your idle is high and it drops down after some time and the smell goes away, thats because your air fuel ratio settles down with the RPM and the cat is working so everything goes nice and clean, but it takes a minute or two for this to happen.
The fuel pressure regulator is hooked upto a vaccum source so when the RPM is raised it raises fuel pressure along with it, so your RPM going up a few hundred raises fuel pressure 3-4psi. It's pretty standard among most cars except for cars with non-return style fuel systems.
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