Explain better how this differs from adding the distributions.
A comparison like
uniform(3,4) - uniform(0,1)
vs
uniform(3,4) .- uniform(0,1)
Could be helpful.
Also note the false intuition 'the distribution of the difference between draws uniform distributions should be uniformly distributed' can be checked by thinking about and plotting
uniform(0,1) - uniform(0,1)
However, that distribution should be triangular, and the simulated distribution in your plot looks somewhat far from this. Why not make that an analytical computation?