Cool .. somebody really looked at my data
Unfortunately I can't remember about the situation where both listener and source are under the layer. This calls for a another test.
As for the bulge .. it could be even bottom bounce, in case that reflection angle would be taken into account (or phase interference). Otherwise the bottom effect would be uniform for all distances. Tests with different water depth would solve this. I recommend you try it, it's great fun, but little time consuming.
I'm quite busy and I invest all spare time in the 'other project' (see my signature).
It's a pity we most probably will never know how exactly DW model works, and it still can surprise.