New to this, but I'll make a guess.
Load capacity would be how much weight you can put on the dolly. You only put one of the two axles on the dolly, so it's how much weight is on that axle. The tow capacity must be the total weight of vehicle being towed plus the weight of the dolly.
The the dolly suspension has to handle the load that's on it and the coupling has to be able to pull and stop the total weight.
I'm sure I'll get corrected if I am wrong.