It seems reasonable to me for
XC▸ to put stations on their map when they are only used for planned diversions and as rail replacement railheads. However, if you look at
their relevant web page you'll see that their confusion about what's where (and Castle Cary in particular) runs deeper than the map Dave spotted.
Online they use a network map without these extra stations, so presumably added them to make the poster easier to follow. But the
online map for April includes the same orange line marked P for 6th and 13th April, and the text beside it is:
P Castle Cary
Saturdays 6, 13 April
The 0608 Bristol Temple Meads to Plymouth will depart a few minutes later and runs as scheduled from Taunton.
Now I suspect you might preferred to head it Weston-super-Mare and say that train doesn't call there...