Matplotlib patchcollection polygons do not touch

I have two polygons which are supposed to touch each other horizontally (no gap).
However, when trying to plot them with PatchCollection, there seem to be a gap between the two, they are not touching:

import matplotlib.pyplot as plt
import numpy as np
from matplotlib.collections import PatchCollection
from matplotlib.patches import Polygon

fig, ax = plt.subplots()
points1 = [[ 0.,  0.],
           [10.,  0.],
           [10., 10.],
           [ 0.,  8.]]
points2 = [[10.,  0.],
           [10., 10.],
           [20., 10.],
           [18.,  0.]]
pc = PatchCollection([Polygon(points1), Polygon(points2)])
ax.add_collection(pc)
ax.autoscale_view()
plt.show()

Is there any reason for that in my code, and can I fix it?
Thank you.