I think we can usefully put Rawls to work here and use his distinction between a concept and a conception (Theory of Justice).
▻ CONCEPT OF MERITOCRACY
The concept of meritocracy does have a pretty uniform, independent meaning. This meaning is captured by Norman Daniels :
I take a meritocracy to be a society whose basic institutions are governed by a partial theory of distributive justice consisting of principles of the following types:
(1) A principle of job placement that awards jobs to individuals on the basis of merit;
(2) A principle specifying the conditions of opportunity under which the job placement principle is applied;
(3) A principle specifying reward schedules for jobs.
(Norman Daniels, 'Merit and Meritocracy', Philosophy & Public Affairs, Vol. 7, No. 3 (Spring, 1978), pp. 207-8.)
▻ CONCEPTIONS OF MERITOCRACY
If there is a reasonably uniform concept of meritocracy, however, and here your point comes in powerfuly, there is a broad variety of conceptions of meritocracy. Views about what counts as job placement according to merit; of what the conditions for opportunity should precisely be; and of what rewards are to be assigned to merit - all these are matters to social context or of system context to follow your own language.
▻ THE OBJECTIVE INDETERMINACY OF MERIT
Not only is there a broad variety of conceptions of merit, there is no objective decision procedure for deciding between them. Take merit schematically as :
S merits X in virtue of M, where S is a person, X a mode of treatment or an outcome, and M some feature possesses. (Richard Fallon, 'To Each According to His Ability, From None According to His Race: The Concept of Merit in the Law of Antidiscrimination', 60 Boston University Law Review 815 (1982), 822, note 1.)
To see the undecidable indeterminacy of merit, consider two cases :
In the first example, 'S merits X in virtue of M', might become: S deserves this particular job, indeed has an indefeasible right to it, when she satisfies better than anyone else the employer's criteria for selection of candidates for the job which involve qualities which are thought to be of general value in the society and are strictly job related, in the sense that they indicate how well someone will be able to perform the core elements of the job, conceived as the production of 100 widgets per hour.
In the second example 'S merits X in virtue of M' might become: S should be
considered for this particular job, and is qualified to be awarded it, provided she satisfies the features determined by society as relevant for carrying out the purposes of the job, broadly conceived to include the ability to work together with others, using criteria for selection which indicate general ability to carry out that purpose, whether contingently or not, and whether those features are thought to be of general value in the society or not. (Christopher McCrudden, 'Merit Principles', Oxford Journal of Legal Studies, Vol. 18, No. 4 (Winter, 1998), pp. 547-8.)
What objective way is there of deciding between these cases on the basis of merit ? No such way, I suggest : and this is within a single social system, to say nothing of deciding in the case of diverse social systems.
Norman Daniels, 'Merit and Meritocracy', Philosophy & Public Affairs, Vol. 7, No. 3 (Spring, 1978), pp. 206-223.
Christopher McCrudden, 'Merit Principles', Oxford Journal of Legal Studies, Vol. 18, No. 4 (Winter, 1998), pp. 543-79.
Richard Fallon, 'To Each According to His Ability, From None According to His Race: The Concept of Merit in the Law of Antidiscrimination', 60 Boston University Law Review 815 (1982).
Michael Young, Rise of the Meritocracy (London: Thames and Hudson,
1958). (Young was the original theorist of meritocracy but the book is a satire that was widely taken to be a defence.)