You feel heavy and it reads more because the poor spring not only has to support you from falling through the floor, but also accelerate you counter to what gravity wants to do.

你感到自己变重以及读数增加,因为现在弹簧不仅要支撑你,不让你掉下来,还要抵消重力的作用让你加速

耶鲁公开课 - 基础物理课程节选