You're right; in this example (and in the painting, I think) the bottom of the body appears to be supported on a level with the hubs of the wheels, with a big space left above the actual dropped axle. Which seems a bit pointless, really.
(Perhaps it simply lowers the weight of the wrought-iron axle itself to a safer level?)

Here's a picture of a tub-cart type vehicle where the hubs are actually in the middle of the body, from Wikipedia...