In reality they haven't got a clue who's coming here or how long they actually stay. The figures are based on questions asked of a small sample of people entering the country. The authorities have no idea whether the people answering their questions are being totally honest or not, they have even less idea how many people who may have come here only intending to stay for a short while yet never returned for whatever reason.
https://www.ons.gov.uk/surveys/infor...engersurveyips
http://www.bbc.co.uk/news/uk-politics-19646459
Anyone would think HMG doesn't really want accurate immigration figures...