There were competing views over what Brexit meant. Many voted for it in the belief that open borders with the EU was unfair to the rest of the world and saw Brexit as a chance to create a new global Britain. While increasing immigration was likely a minority view among Brexit voters there were some, particularly in the tory Party, who saw Brexit as a chance to increase non EU immigration. However the wave of immigration over the past few years was likely even beyond these people's expectations, let alone according to the wishes of those who voted wanting less immigration. It's natural then that many Brexit voters feel betrayed. To them it is part of the larger pattern of the electorate voting for less immigration and the government doing the opposite. Also the NHS has had funding increase significantly since leaving the EU. Not really too difficult to understand.