The different result when I set `vmin` and `vmax` with different plot type

Question

Background

For plotting 2-d spatial distribution of some attribute, contourf and pcolormesh are both good choice as a visualization form.

Due to the actual meaning of the plotting data, I want to set the value nearby zero are showing in white color(for example, [-0.2,0.2]); and zero value should be the midpoint

From the question [Defining the midpoint of a colormap in matplotlib, I tried to set vmin and vmax to adjust the colormap.

Data import

import pygrib
grib='fnl_20141110_12_00.grib2';
grbs=pygrib.open(grib)
grb = grbs.select(name='Vertical velocity')[8]#Vertical velocity
data=grb.values
lat,lon = grb.latlons()

1. Contourf plot

m = Basemap(llcrnrlon=110,llcrnrlat=34,urcrnrlon=122,urcrnrlat=44,projection='mill')
fig=plt.figure(figsize=(8,4))
ax1 = plt.subplot(121)
x, y = m(lon, lat)
cs = m.contourf(x,y,data,cmap=plt.cm.RdBu)

cb = m.colorbar(cs,"bottom", size="5%", pad="8%")
ax1.set_title("Original",fontsize = 15)

ax2 = plt.subplot(122)
cs = m.contourf(x,y,data,cmap=plt.cm.RdBu,vmin = -1.8,vmax =1.8)
cb = m.colorbar(cs,"bottom", size="5%", pad="8%")
ax2.set_title("symmetrical vmin & vmax",fontsize = 15)

2. Pcolormesh plot

m = Basemap(llcrnrlon=110,llcrnrlat=34,urcrnrlon=122,urcrnrlat=44,projection='mill')
fig=plt.figure(figsize=(8,4))
ax1 = plt.subplot(121)
cs = m.pcolormesh(x,y,data,cmap=plt.cm.RdBu)
cb = m.colorbar(cs,"bottom", size="5%", pad="8%")
ax1.set_title("Original",fontsize = 15)

ax2 = plt.subplot(122)
cs = m.pcolormesh(x,y,data,cmap=plt.cm.RdBu,vmin = -1.8,vmax =1.8)
cb = m.colorbar(cs,"bottom", size="5%", pad="8%")
ax2.set_title("symmetrical vmin & vmax",fontsize = 15)

I upload the data here as an .grib2 file. If you are interested, you can download it

My doubt

With the contourf plot type, setting symmetrical vmin and vmax couldn't adjust zeron to the midpoint of the colormap. That's different from pcolormesh.
The same colormap RdBu_r ==> different result in contourf and pcolormesh. In pcolormesh plot, the middle partof colorbar are shown in white color. But in contourf plot, the white color seemed to be ignored.