A lot depends on turnout. Corbyn’s party actually polled more votes in both 2017 and 2019 than Starmer’s party did in 2024 but the low turnout meant Starmer was able to not only win but win with a huge majority.
It seems clear that a lot of people who voted Conservative in 2017 and 2019 either voted for Reform in 2024 or (more likely) stayed at home.