You have altered the waterfall window from its default setting. It looks like you're using a Tukey 0.25 for the Spectral decay Left Window. The Spectral Decay is the window used when a waterfall is generated. The left should be a Hanning Window and the Right should be a Tukey 0.25 window. Once changed, regenerate the waterfall.
If you want to return REW to all its default settings, execute a Delete Settings and Shutdown in the Settings pull down.
When you post a waterfall of a sub, please use the same axis as use when you post a response graph. Use the standard Vertical graph axis of (45dB - 105dB) and the Horizontal graph axis of (15Hz - 200Hz) using the Graph Limits button in the top right corner of REW.