The caster angle is fixed the rigid front axle housing. There is no
adjustment and the only way the caster angle can differ from side to side
is either the axle housing is twisted or the swivel ball attachment has
elongated bolt holes (unlikely unless it has been badly neglected) - or the
swivel bearings are worn, which they should have noted if this was the
case. It is possible the caster angle difference is road damage, but it may
be manufacturing tolerance. The overall caster angle for both sides is
controlled by the radius arms and worn bushes will allow it to vary. It
will also vary with spring deflection, so it is just possible that a change
in load between when the two sides were measured would cause the
difference. Because the caster angle changes with spring deflection, I am
surprised the tolerance is so close.
Adjustment would be by replacing components, in this case probably the axle
housing, but I would investigate more closely before doing anything that
drastic.

They are correct - it it not adjustable within any normal meaning of the
word, and is probably a manufacturing tolerance issue.  I'd not be
worrying at all about a caster error of less than about 0.7° - it'll
make no noticeable difference to either handling or tyre wear.

Note:  I'm a mechanic by trade and have done more wheel alignments than
I can remember, but the main thing I do remember is the trouble I had
trying to convince clients that an alignment "error" is neither
significant nor cause for concern.