The lines produced by
cairolatex pdf for fill patterns 4, 5, 6 and 7 are not continuous and create the impression of misalignment at the edges.
Fill patterns 1, 2 and 9 on the other hand look perfectly continuous and smooth as do all the aforementioned fill patterns in the default terminal.
To see this, take a look at the pdf file produced by
set terminal cairolatex pdf
set output "test.tex"
Gnuplot Version 4.6 patchlevel 3