People in Jerusalem reason that they know where Jesus comes from, whereas 'when Christ cometh, no man knoweth whence he is.' Their expectation may sound surprising beside other Gospel passages that associate the Messiah with Bethlehem.
The difference is evidence of varied messianic ideas rather than a contradiction that must be forced into one scheme.
Jewish hopes concerning the Messiah drew upon many scriptural texts and interpretive traditions. Some expectations emphasized Davidic descent and Bethlehem; others included hiddenness, sudden manifestation, or uncertainty about the Messiah before his public revelation. Surviving sources do not support the idea that all Jews held one standardized messianic profile.
John uses the crowd's claim ironically: they believe they know Jesus' origin, while the Gospel insists his ultimate origin is with the Father.
The historical diversity helps readers avoid treating every statement by a Gospel crowd as official Jewish doctrine. Different speakers can hold different expectations.
John exploits those expectations to deepen his recurring question, 'Where is Jesus from?' Human geography supplies part of the answer, but his true origin, in the Gospel's theology, is heavenly.