Smooth color rendering for surface pressure but not other variables?

[docgregt]

When I render surface pressure I get a nice smooth rendering using the following parameters:

Sea level pressure:
https://i4ncwms.i4-insight.com/wms?FORMAT=image%2Fpng&TRANSPARENT=TRUE&STYLES=default-scalar%2Fx-Sst&LAYERS=forecast%2Fssp&TIME=2019-03-03T21%3A00%3A00.000Z&COLORSCALERANGE=95320%2C105700&NUMCOLORBANDS=250&ABOVEMAXCOLOR=0x000000&BELOWMINCOLOR=0x000000&BGCOLOR=transparent&LOGSCALE=false&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&SRS=EPSG%3A4326&BBOX=118.60266666667,-28.602666666667,149.30133333333,2.0959999999999&WIDTH=256&HEIGHT=256
To get:

But when I try to render windspd with same color map (or any color map) I get a very pixelated render using basically same parameters:
Windspd:
https://i4ncwms.i4-insight.com/wms?FORMAT=image%2Fpng&TRANSPARENT=TRUE&STYLES=default-scalar%2Fx-Sst&LAYERS=forecast%2Fwindspd&TIME=2019-03-03T21%3A00%3A00.000Z&COLORSCALERANGE=-1.206%2C27.3&NUMCOLORBANDS=250&ABOVEMAXCOLOR=0x000000&BELOWMINCOLOR=0x000000&BGCOLOR=transparent&LOGSCALE=false&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&SRS=EPSG%3A4326&BBOX=118.60266666667,-28.602666666667,149.30133333333,2.0959999999999&WIDTH=256&HEIGHT=256

to get:

How can I get other variables to render as smoothly as the surface pressure?

[guygriffiths]

We always use a nearest neighbour method for translating from data to images, and never interpolate. Looking in detail at those images, they are at the same zoom level, and the individual data points are 6x6 pixels on both images. I'm pretty sure that the surface pressure only appears smoother because the data is smoother