Why do these psychology textbooks write 'differential' as an adjective, rather than 'different'?

Jul 2021
Diamond Bar, California
Are these books using 'differential' correctly? Why not just write 'different'? English isn't my first language, and I don't understand the difference between 'differential' and 'different' as adjectives.

1. Zoltan Dornyei. The Psychology of Second Language Acquisition - Oxford Applied Linguistics. Anyone know the page #?

Differential success: the most obvious difference between the
attainment of an Ll and an L2 is that the former is
invariably considered successful by the learners themselves
(i.e. we do not hear people complaining about not speaking
their mother tongue well enough), whereas most L2
learners never reach as high a level in the target language
as they have originally hoped for. The issue of differential
success is therefore central to any SLA theory construction,
giving rise to the theory of Universal Grammar (See Ch. 3)
and the Critical Period Hypothesis (See Ch. 6).
2. Mario Bunge, Ruben Ardila. Philosophy of Psychology. Anyone know the page #?

Differences among social classes have been studied by psychologists
and found to be very important. Attitudes, community involvement, val-
ues, and even family interactions vary with social stratum. Violence is
more common in lower social classes. Personal happiness is associated
with education (as well as with personality factors). High-status persons
tend to speak more and to be listened to more often.
One complex problem in this domain is the interaction of socioeco-
nomic status and intelligence. A positive correlation has been found be-
tween the two that is stronger for adults than for children. In the contem-
porary controversy over the relative influences of genetic and
environmental factors in the origin of intelligence, social class has played
an important role. Perhaps middle- and upper-class people are more intel-ligent because intelligent people tend to move up the social ladder and less
intelligent ones tend to move down.
In any case, socioeconomic status is an important index of the social
matrix. An upper-class person in Paris has much more in common with an
upper-class person in Los Angeles or Tokyo than with a lower-class per-
son in the same city. Values, norms of conduct, lifestyles, and philoso-phies of life are very different across classes. Perhaps in the future there
will be no social classes, but the differential prestige associated with
different occupations, differential access to power, and different social
roles will probably remain.
3. Critical Psychology: An Introduction edited by Dennis Fox, Isaac Prilleltensky. p 238.