As most always on stuff like this only YOU can tell us what the difference is! You see, if you used the same exact paint to paint both things and one of them looks different than the other YOU did SOMETHING different! There is no other explanation, it is pure and simple logic, SOMETHING was done different.
Be it, more or less air pressure and it is solvent pop. It could have been laid on too wet so some toner in the color (most "Blacks" are made from a formula like any other color) came to the top. Your substrate color of the bed was different (different primer color). There isn't the same amount of coats and you are seeing the primer thru it. You didn't overlap as much and it is thin and you are seeing the primer thru it. You moved faster, again, it is thin and you are seeing the primer thru it.
SOMETHING was different in the application on those two parts, think long think hard, you did SOMETHING different.
Edit, I just re-read your post. You shot a DIFFERENT GALLON of the "same paint", that makes all the difference in the world. First off, this doesn't change the fact that you need to look at all the things YOU did as I pointed out to count out all factors. If all those factors are the same, then it MUST be the paint, right? Just using logic, if everything you did was the same on both cab and bed then the only variable is the paint it's self.
And as I write in the "Basics of Basics" on choosing a color and buying your paint. (Click here)
Tip 5. After you have your color picked for goodness sakes donít be a cheapie when buying your paint. Figure out how much youíll need for the whole job. We are talking every thing you plan on painting, outside, inside, dash, jambs, trunk, everything. When you have an idea how much, add at the very least 20% more. If one gallon is enough, buy another quart. Buy all the paint you will need before you start painting anything. Get a few extra gallon cans and use them to intermix ALL the paint. You then have all the paint you need, no mismatched parts, no running out, you are set to go. If you have a that quart left over when you are done, so what? Running out of paint is NOT pretty, it is a disaster in many cases. Now, why intermix? This is a VERY painful lesson you donít want to learn the hard way. This is it in a nutshell, if you were to go to the paint store and have three gallons of the same formula mixed you would end up with three different colors! I will bet you a dollar, here is why. Some toners are very strong, just a drip will change the color. A couple of different people could mix them, some people mix better than others. There are other variables such as one toner used gets emptied and the next toner used has more solvent in it because it is new and has less strength. Now, these colors may not be ďthatĒ different. If you were to paint three different cars with those gallons you may not even see it. But if you were to paint your hood, fenders, and quarters with the three different gallons you sure would! I repeat, this is a VERY painful lesson you donít want to have to learn the hard way, BUY ALL YOUR PAINT UP FRONT.