One common linguistic treadmill is <plural count noun for small items> → <singular mass noun for stuff consisting of those items> → <singular count noun for a specific quantity of that stuff, in other words for a specific group of those items>. The clearest example in English is ‘agenda’ (originally the plural of ‘agendum’, an obsolete word in place of which we now use ‘agenda item’), which has now generated its own plural. Other examples: ‘algae’ (still the plural of ‘alga’ to biologists but a mass noun to layfolk) and ‘virus’ (which went backwards when we learnt more about the subject; originally a mass noun, now a count noun with a plural).

Any specific usage can be justified on its own terms, but the whole process is the old philosophical problem of distinguishing a set of things from the things themselves.