In general, the number of visas that embassies and consulates can grant in a given period of time is limited by certain quotas. Diplomatic ties between nations, econromi modernomic factors, & evolving immigration laws can all have an impact on these quotas. Priority is frequently granted to particular applicant categories according to urgency or particular requirements. Students applying to programs with impending start dates, for instance, might be given preference over those with later start dates.
sj