Police may give small detail regarding why they are interviewing you, however, they do not have to give you all the information they have before the interview takes place.
For example, the police may advise you that you are being interviewed for a shop theft, but they may not advise you until the end of the interview that they have CCTV video footage of the shop theft. This is a commonly used interview tactic by police as answers given in a record of interview can be used as evidence.
This is why it is important for all people to exercise their rights to contact a legal representative prior to the interview. A lawyer will ask a series of questions to a police officer regarding the offending and the evidence they have prior to giving interview advice.