The rate of return of unused numbers may fluctuate from month to month, just as demand may fluctuate. Lower returns mean fewer numbers available for subsequent reallocation. Fluctuations can cause cut-off date movement to slow, stop, or even retrogress. Retrogression is particularly possible near the end of the fiscal year as visa issuance approaches the annual limitations. The annual per-country limitation is a cap which visa issuances to any single country may not exceed.
Applicants compete for visas primarily on a worldwide basis.
The country limitation serves to avoid monopolization of virtually all the annual limitation by applicants from only a few countries. This limitation is not a quota to which any particular country is entitled, however. Clearly all countries could not receive up to their numerical limit since the worldwide annual limitation would not permit visa issuances anywhere close to such a total. This oversubscription may require the establishment of a cut-off date, which is earlier than that which applies to a particular visa category on a worldwide basis. The prorating of numbers for an oversubscribed country follows the same percentages specified for the division of the worldwide annual limitation among the preferences.