This is how AlphaToCoverage works. Your opacity mask becomes an MSAA coverage value, which allows anti-aliasing of the mask.
To get a result closer to the deferred renderer, you can make your OpacityMask input into a steeper gradient. For example if you have an OpacityMaskClipValue of .33, you can do a Mul with 5 and then Subtract (.33 * 5). This will make the OpacityMask gradient steeper, pushing the translucent bands closer together, but keeping the value .33 in the same spot.