where does pyplot define the number of levels

Question

I'm trying to understand how levels, values, boundaries are created in the contourf-toolchain.

import matplotlib.pyplot as plt
import matplotlib as mpl

t=np.array([[-5,0,5,0,0,0],[0,0,5,0,0,0],[0,0,5,0,0,0],[5,5.2,10,5,5,0],
            [0,0,5,0,0,0]], dtype=np.double)
print ("print as is")
print (t)
print ("appearing in a contourf as:")
print (np.flipud((t)))
# assuming indices: array[x,y]
# value 10 supposed to be at 0,0
# contourf's pos. x-axis is pointing up!! 
xmin, xmax=-2, 3
ymin, ymax=-3, 1

fig, axs = plt.subplots(nrows=1, ncols=2)
ax0 = axs[0]
ax1 = axs[1]

# plot and add colorbar
set0 = ax0.contourf(t, extent=(xmin,xmax,ymin,ymax), cmap="brg")

#colorbar with coarse steps
cba = plt.colorbar(set0,ax=ax0)
# # This makes the colorbar "appear continuos"
# cba.boundaries=None
# cba.values = None
set0.changed()

# define colorbar and then plot
cNorm = mpl.colors.Normalize(vmin=np.min(t), vmax=np.max(t))
sm = mpl.cm.ScalarMappable(cNorm, cmap="brg")
cbb = plt.colorbar(sm,ax=ax1)

set1 = ax1.contourf(t, extent=(ymin,ymax,xmin,xmax), cmap="brg")

plt.show()
print("left .boundaries: {:s}".format(str(cba.boundaries)))
print("left ._boundaries: {:s}".format(str(cba._boundaries)))
print("right .boundaries: {:s}".format(str(cbb.boundaries)))
print("right len(._boundaries): {:s}".format(str(len(cbb._boundaries))))

For the left plot the colorbar has 9 boundaries and 8 discrete color values for the right one there are 257 boundaries. I did a couple of vars(...) and dir(...) on the various objects. Where is decision made on the amount of boundaries and values made? What is the deviation of the right plots colorbar creation from the default behaviour?