Note that these aren't polls taken in each riding. 338 takes the voting data of the riding in previous elections and then uses national and province-wide polls (which are broken down by voter demographics and how people voted previously) to predict how voter intention is changing. So for example a middle class white riding that voted mostly NDP previously is going to be predicted more Liberal when Canada-wide polls are showing that the Liberals are polling well with the white middle class and former NDP voters.