Because the libtards and their ridiculous political correctness shaming and demonizing those 'far right' folks who do not share their 'progressive' views on immigration, environment, the economy etc, they tend to keep their true political inclinations hidden when being interviewed by pollsters or during polite social gatherings. Even the neutrals and the independents. It's for self-protection.
Some pollsters, usually bribed, also use polls to shape a narrative, and what they often do is oversampling a particular demographic. It also keeps those mentally ill libtards happy by letting them know they're 'winning'.