How to fix a floating wall shelf on an uneven wall
Solar Mike provides one excellent option of cutting off the back edge of the shelf that sticks out.

If you don't have a saw (or the ability to cut a reasonably straight line) you could shim out the "short" shelf so it sticks out as far as the other one. 

As an added bonus, it appears that there is some trim below the shelf that would hide the thickness of the shims so they wouldn't be obvious from below!

Based on the update to the OP that includes pictures of the mounting systems, I'd suggest that the best method would be to attach a wood shim to the wall behind the short shelf, then mount the shelf to that.

I'd guess that the gap is about 1/2" so adjust based on actual measurements. 


Get/cut a strip of material 1/2" thick.


Many big-box hardware stores will cut lumber for you if you purchase it there.)

Attach this to the studs in the wall using screws of the same approximate size (bigger will never hurt!) as those that came with the shelf kit.


Note that these screws are an additional purchase, get them at the same time as the shims themselves.

Attach the mounting screws to this strip of wood
Attach the shelf to the mounting screws.


You might be able to get away with replacing the screws that came with the shelf with longer screws that are the same diameter. (For example, if those are 2" #8 wood screws, get 3" #8 wood screws. Take the screws to your local hardware store and compare them to what they have in stock to find out what size they are.) I say might because you're going to have the shelf hanging farther from the wall than it's intended to. This makes it a longer lever and amplifies the stress that the weight of items on the shelf will apply to the screws in the wall. For a couple of stuffed animals/figurines, you'll probably be fine - books, not so much.

Once you've got the longer screws in the wall, leave them sticking out far enough to accommodate the shims and the extra needed for mounting. This may take some fiddling to get them to just the right depth to accommodate the shims/mounting, but still be snug enough for a firm mount.

You could still scribe the back of the longer shelf to match the wall, but that's likely going to shorten the space for the long support rods for that shelf, so you'd need to trim a little bit off the length of those to match. If you don't have the proper wood cutting tools, you probably also don't have the proper metal cutting tools.

Note: I'm assuming you don't have the proper tools, please forgive me if that's an incorrect assumption.