There actually is a Falston (Faulston) in Wiltshire. Do you suppose there might have been confusion in the mind of the person who completed the documents? Henry himself would have been illiterate. If he signed on at Plymouth and someone had said "Is that near Plymouth?" he might have said "Sure".
I have a couple of Wiltshire ancestors who went into the military around the same time and ended up elsewhere (Kent and Devon) afterward.
Doing a general census search for people born in Faulston produces these results (I haven't checked images):
1871 (no county of birth recorded)
O R Antony Bushell c1854, Seaman on vessels, Durham
Mate John Penfold c1840, Mate on vessels, Durham
1861
Jonathan Roebuck c1811, born Faulston, Yorkshire
1901
James Jeff c1845, Faulston, Kent
A search for birthplace Falston -- all years, specifying no other info -- produces numerous results in all years 1861-1911. Specifying Devon, there is one result in each of 1901 and 1911. In 1911 there are 13 results for Falston altogether, with various other details, but only one is Devon. In 1891, that person is:
Emily Francis Harris c1889, born and living in Talaton, Devon (Honiton reg dist).
and indeed, the later "Falston" is a mistranscription, although it looks like Falaton. She was in Broadhembury in 1901.
Where did he sign on -- was it in or near Plymouth? If it was elsewhere, "near Plymouth" might just have been a stand-in for "we have no idea but it's in Devon somewhere".
There are several Shepard/Shepperd/Shepherds in the censuses in Talaton, Devon, in several years, in case they're of interest. In particular, there is a Robert Sheppard born c1776 in Talaton in 1841.
Just some random thoughts you might want to think about.