A practical difference between the abovementioned reasons would be whether a
tzuras hapesach would also need to be built within three
tefachim of a wall. The first reason that there cannot be a space that would allow an animal to squeeze by would also apply to a
tzuras hapesach. However, the other reasons are not applicable to a
tzuras hapesach. A
tzuras hapesach is an actual
halachic barrier, it does not merely serve as a reminder not to carry into the
reshus harabim (reason #2). It also does not need to be assisted by the other walls (reason #3).
[
משנ"ב שסג, כג, וראה שעה"צ טז; ביאורים ומוספים דרשו, 23-20, וראה שם, 24]