This is because the high and low thresholds of the 555 internal comparator are determined by the three internal resistance partial pressures when Uco is not added, that is, the high and low comparison thresholds are 2/3Vcc and 1/3Vcc respectively. As can be seen from the internal circuit diagram, after adding Uco, the high threshold becomes Uco and the low threshold becomes 1/2Uco. Obviously, the high and low thresholds are not only changed, but also controlled by the external voltage Uco.