A 5% drop-out rate implies that each neuron is "on" (not dropped) with probability 0.95. For a small number of neurons, you can expect that all of them will be "on" with high probability. This is why your curve is mostly smooth with some spiking: the spikes correspond to the (somewhat rare) cases where one or more neurons is "off".
Moreover, because the dropout is so small, it's likely that the network is trained in such a way that it "expects" that all of the neurons are present -- so losing one or more neurons is reducing the network's effectiveness.