The world moved and the model did not. A model is a snapshot of relationships learned from historical data — when the world underlying those relationships changes, the model's assumptions quietly stop holding, and performance degrades in a way no error message will ever announce.
There are three clocks in a system running an RTOS and it is worth separating them before writing a single delay. There is the hardware timer, counting a crystal-derived clock in silicon, which does not know or care what software is doing. There is the tick, a periodic interrupt that increments a counter and is the only time the kernel can see. And there is the task's own experience of time, which is the tick as filtered through whether that task was actually running when something expired.